Flate Rate shipping multiple products

I'm trying to set up my shipping. We are only going to be using USPS Flat Rate. The thing is, that I can fit multiple quantities in one box to save the customer on shipping cost, Instead of just adding flat rate fee/item. I'm not sure on how I should go about this.

Maybe set items to a specific category like, 1 for small flat rate box. And set a max quantity of product to put in one box, like 3, then it would add a new small flat rate box.

Suggestions/help please.

This USPS mod will let you do that

USPS Shipping Module Update - RateV4 - IntlRateV2

It has a "Filter by Product Volume" or "Filter by Weight" function built in.

Here is an example of how we use it which sounds similar to your needs:

A small flatrate box will hold 67 cubic inches. We have a product that which we can fit a quantity of 5 in a small flatrat box. So, we use 13 cubic inches as the volume of that product. If a customer has 5 or less of that product in the cart, then the small flatrate box is an available option. If a 6th is added, then the small flat rate box is no longer an option and the customer is required to use a Large Video Mailer box, Regional Flat Rate A or a Medium flat rate box. More than 8 will not fit into a Large Video mailer box, so it is automatically removed from the available options when the 9th item is added to the cart leaving the Regional FlatRate A box and the Medium FlatRate box.

We have based the volume of other items that we sell to cause the smaller boxes to be removed as necessary.

We also do not wish to ship the product described by firstclass mail, so we modified the coding so that 12.5 cubic inches is the max that we will ship firstclass. If the example above is added to the cart, the firstclass option is automatically removed but other items of lesser volume are still allowed the firstclass option.

The module does require a SQL patch to be applied to the database so that digits 6-11 in the weight field can be utilized for the product volume. That's listed in the instructions.

OK, Installed the module and the SQL patch. Where would I enter the products volume? Under weight?

Ahh read the instructions. But is the max volume already set for small flatrate and medium?

Also, nothing is coming up under Estimate shipping.

Did you enter a volume for each product using the format indicated in the readme file?

Yes, the Max volume is determined by the physical dimensions of the various USPS boxes and coded into the addin. The volume of each product must be input by you using the format indicated. IF there are 2 different boxes for a particular flatrate class, the larger of the two volumes is coded as the max.

My realworld example is our Product A which weighs 0.1 pound and has a volume of 13 cubic inches is entered into the weight field as

0.100000013

Copied from the ReadMe Instructions

Volume Filtering

This allows you to control shipping service quoting independently of product weight. One benefit is that some products may be large but light or vice versa. You can customise shipping quotes based on the products or product attribute weight.

You must apply the SQL patch described above in the install instructions. Then you add extra information to the products weight field.
You specify a weight for 1 unit of a product in lb.
Eg: Product A weighs 0.0625 (lb)
So volume (cubic inches) is defined by digits 6-11 after decimal point.
eg: Product A is a big inflated balloon weighing 1oz and is 6859.3 cubic inches.
Products A's weight is set to: 0.06250685930
USPS will not ship anything much larger so this impementation should cover all posibilities. The above also works for product attribute weights.

Sometimes the extra digits can be confusing.

If you turn XML debug on at the bottom of the setup screen, the details of what is happening will be displayed at the top of the Shipping Estimator Popup window. Included in the display will be the type of filtering that you've enabled just in case the wrong one was fatfingered.

Once satisfied, remember to turn off XML debug.

I put a correct number in, but nothing still comes up in the estimator window. I have XML debug on, I don't know what's going on. Can you give my site a look? Add the ashtray fix (spring only) in the cart.

http://www.ponyrparts.com/

Quick look shows that it seems to be functioning as expected.

Once you enter a zipcode (USPS doesn't know where you're sending it for an estimate until you enter a zipcode), you will see postage options for:
FlatRate Envelope ($4.95)
FlatRate Small (5.20)
FlatRate Medium ($10.95)

I know that zipcode doesn't matter for the regular flatrate boxes but other options are zipcode dependent; thus the requirement (I assume)

It appears that you have 3 pounds set into the "Tare" weight field, so zencart is adding 3 pounds to the total product weight for a package weight of 3.1 pounds. That will cause first class packages to be removed but shouldn't affect the flatrate boxes.

You can edit the Tare weight in Admin/Configuration/Shipping&Packaging 4th item down the list.

Here's a link to the official support thread for the module where Marco, the developer can offer more detailed assistance. It looks like you're on the road in the right direction.