My ZenCart not submitting price to PayPal

I've been having this issue for a few months, after the last upgrade to the store.

Basically if PayPal payment is chosen as the payment method then it takes you to PayPal but with no price entered.

I've checked the form data submitted and ZenCart is setting the price to 0 for a reason I can't work out.

See post data:

<input type="hidden" name="item_number" value="Store Receipt" />
<input type="hidden" name="upload" value="1" />
<input type="hidden" name="amount" value="0" />
<input type="hidden" name="shipping" value="0.00" />
<input type="hidden" name="tax" value="0.00" />
<input type="hidden" name="tax_cart" value="0.00" />

What I've tried so far:

Reuploading all files to the store except the config files and images
Removing then reinstalling and reconfiguring the PayPal module

I'm out of things to try, can someone please help with this or at least point me in the right direction?

What currencies do you have defined in your Admin->Localization->Currencies?
Which is your default?
Which currency are you doing your shopping in?
Does this affect all orders, or just some?
Does this apply only to taxable orders?
What shipping method is being used for these orders?
Are any coupons/discounts/gift certificates being used?

You said "since the last upgrade to the store" -- which version is your store at now?
What Zen Cart patches have you installed onto that version?
What versions have you upgraded from?
What addons/contributions have you installed?

Thank you!

It took looking at the defined currency to realise the currency code was wrong. Many thanks, I've spent hours on this without evening checking that page, doh...