如何上传包含单个付款项目和分期付款项目的购物车

I wrote my own shopping cart [in PHP] a couple of years ago that uses PayPal's Classic API "Payments Standard" -- specifically the "upload" command to send the contents of the shopping cart to PayPal. Now I want to add the option to pay for an item with installments and I want that to integrate with the shopping cart process.

In other words, I want the customer to be able to put Item A into the cart and select a single payment option, and then also put Item B in the same cart but select an installments option for that item. Then, when they hit the "Check Out With PayPal" button, I want the Item A and Item B to be processed in the same transaction. My cart also has a feature that allows the customer to enter a coupon code.

I have spent hours running in circles though the PayPal API documentation and I can't find a solution that seems to cover everything that I need:

1. No monthly payment (I'm too small to absorb monthly payments, so it must be a fee-per-transaction service).
2. Ability to process single payment items AND installment items from the same cart.
3. Ability to accept a discount/coupon and apply it to the transaction at the PayPal side.
4. All collection of payer's shipping address, payment information, etc, done at the PayPal side.
5. Determine if sales tax is required at the PayPal side.
6. Provide an IPN type response.
7. I'd rather not install code on the server [that needs to be maintained or reinstalled if I switch hosts].

Can anyone point me to a solution that could cover all of this?

I was looking at the Adaptive Payments solution but it looks like it I would have to determine whether to charge sales tax (and perhaps have to collect the payer's shipping address info, which I want to avoid). I like everything else about Adaptive Payments. Does anyone know if it will be as "nice" about handling the payer's information, as "Payments Standard" does?