BUY is the best verb in an ecommerce producer/consumer product context, but not all payments thru the web fall under this category. Here are some examples that argue for the verb PAY:
- paying a bill to your healthcare provider
- sending a donation to an organization
- putting a deposit to reserve a product
- renting a car
- paying a bill to your healthcare provider - sending a donation to an organization - putting a deposit to reserve a product - renting a car