Need help understanding how to test PayPal Webite Payments Pro

I followed all the instructions (I think - see details at end of this post). In my shopping cart, there is a "Check out with PayPal" button in addition to the usual "go to checkout" button. I click on the PayPal button. During testing, at this point, I've actually had three different outcomes:

1. A screen comes up with a link to login to PayPal Developer Central. I click on that and login. Now I'm at the home page of the "PayPal Sandbox". (Previously, I did set up buyer and seller test accounts.) What am I supposed to do now?

2. Went back and tried again (same browser session?) - got an error in a PayPal sandbox window saying "Return to Merchant" and:

This transaction is invalid. Please return to the recipient's website to complete your transaction using their regular checkout flow.

3. Cleared out browser's cache and tried again: clicked on the PayPal button nothing happened. Clicked on it again and now I'm in "Step 1 of 4: Shipping" screen. (Yes, this is true; I did it again just to check. I had to click on the button twice.) I'm using COWOA exclusively. The shipping info screen has a shipping address for "Joe Smith" in San Jose, CA. I've never entered this address so it must be coming from some test data somewhere? I continue to the Payment screen: It shows my order total but nothing indicating that I'm about to make a PayPal payment and no "choices", just a ""continue checkout" button. I am able to continue all the way to "order complete". But nowhere was I every asked to enter my PayPal account info. In one of the debug emails, I see:

[payer_email] => [email protected]

and the name and address is Joe Smith.

The first debug email (of many) has a subject of "getOverrideAddress - 4" and in it it says:```
not logged in and not in markflow mode - nothing to override

The final debug email for this transaction has a subject of "PayPal Debug log - DoExpressCheckoutPayment, and contains this:

2009-01-31 09:55:29

(sandbox transaction) --> https://pilot-payflowpro.verisign.com/transaction
Request Headers:
Array
(
[0] => Content-Type: text/namevalue
[1] => X-VPS-Timeout: 45
[2] => X-VPS-VIT-Client-Type: PHP/cURL
[3] => X-VPS-VIT-Integration-Product: PHP::Zen Cart - Payflow Pro
[4] => X-VPS-VIT-Integration-Version: 1.3.8a
)

Request Parameters: {DoExpressCheckoutPayment}
Array
(
[ITEMAMT] => 20.00
[SHIPPINGAMT] => 8.95
[AMT] => 28.95
[L_NUMBER0] => TMM001
[L_QTY0] => 1
[L_NAME0] => Meat or Motion T-Shirt Size: Medium
[L_AMT0] => 20.00
[L_TAXAMT0] => 0.00
[SHIPTONAME] =>
[SHIPTOSTREET] => 111 Main St.
[SHIPTOSTREET2] => Apt #301
[SHIPTOCITY] => San Jose
[SHIPTOSTATE] => CA
[SHIPTOZIP] => 95100
[SHIPTOCOUNTRYCODE] =>
[BUTTONSOURCE] => ZenCart-ECGW_us
[CURRENCY] => USD
[TOKEN] => EC-D99WCNVU1RDCHEQ6O
[PAYERID] => 1234567890123
[ACTION] => D
[TENDER] => P
[TRXTYPE] => S
[REQUEST_ID] => 1233413728
[USER] => myusername
[PWD] => ************
[PARTNER] => PayPal
[VENDOR] => myusername
)

Response:
Array
(
[RESULT] => 0
[PNREF] => E78P1AA15E13
[RESPMSG] => Approved
[AVSADDR] => Y
[AVSZIP] => Y
[TOKEN] => EC-D99WCNVU1RDCHEQ6O
[PAYERID] => 1234567890123
[PPREF] => PD07I5MK4A7OE924E
[CORRELATIONID] => 3a3204997455r
[FEEAMT] => 0.33
[PAYMENTTYPE] => instant
[PENDINGREASON] => completed
[REQUEST_ID] => 1233413728
[CURL_ERRORS] =>
)

What is the "normal" behavior supposed to be when I click on the PayPal button in the shopping cart while testing? 

Does PayPal checkout work when checking out without an account?

I have Debug Mode set to log & email - I'm getting the emails but wish I could figure out where the actual log file is!

=======================================
Here's how I configured PayPal Website Payments Pro in Zen Cart:
I already had a business PayPal account, so I went into it and setup API Access, as instructed, and got all the API credential stuff. I then installed and enabled "PayPal Express Checkout" first, as instructed. Then I installed and enabled "PayPal Website Payments Pro".
I also set up a developer account with PayPal, got into my sandbox and set up seller and buyer test accounts.

Moderator's note - Post moved to this froum.

Please do not cross post. Thanks.

I was able to successfully test a credit card payment, FYI, by going through the whole checkout process. The transaction showed up in my test area of my PayPal Manager account.

So - credit card stuff is working ok.

I then tried to do another PayPal payment by going through normal checkout (instead of from the button in the shopping cart). On payment screen I selected "PayPal" then "continue".

I went all the way through to "Confirm Order".

Clicked on "confirm order" and got the PayPal "Return to Merchant" error described above:

This transaction is invalid. Please return to recipient's website to complete your transaction using their regular checkout flow.

I'm at a complete loss as to what this error is all about and would greatly appreciate some help.

FYI: "PayPal Mode" for PayPal Express Checkout is set to "PayFlow Pro - US".

I tried changing it to "PayPal", but then my Website Payments Pro looks like it's going to use PayPal instead of PayFlow Pro.

I'm very confused about what this option is supposed to do.

You'll save yourself a lot of confusion if you stay away from the PayPal Sandbox site which is designed for developers to use.
Doing real live transactions will show you the same results ... and the configuration/setup is much more straightforward.

I took Dr. Byte's advise and gave up on the sandbox. Configured both Express Checkout and Website Payments Pro with Payflow-US in live mode. Was then able to use both PayPal and Credit Cards successfully.

One other note: for Credit Card payments, putting the WPP into Sandbox mode does have some value. You can use a test credit card number and it does work and you can look at your transactions in PayPal Manager by running reports against the "test" rather than "live" server.

But definitely stay away from sandbox for the PayPal payments. I did all my testing live for that (I have both a business and personal PayPal account). I just reversed the sales by applying a refund in Zen Cart. All worked fine.