I recently did some work for a small local non-profit group to accept payment over PayPal. They have multiple, but limited, offerings that they were selling to raise funds, and customers could purchase in variable quantities. They originally used the buttons auto-generated by PayPal, but that functionality forces their customers to go back and forth between the single-page sales page and the PayPal shopping cart, which creates too much complexity for their simple sales process. Below is a simplified version of what I put together for them.
First, this is the sales form:
The next page grabs the values passed in from the previous page and displays a confirmation screen for the customer to review; if all looks ok, the customer will submit the following form. Note the specific form action and hidden inputs (cmd, upload, business, and currency_code) required by PayPal. Multiple products is achieved by creating sets of inputs (item_name_*, quantity_*, amount_*) for each product, with the asterisk being a number; the numbering must start from 1 and subsequent products must be in order, ie. 2, 3, 4, etc., thus the use of the IF statements and the $itemCnt counter variable. The amount_* field represents the unit price of the associated product, not the line total.