Paypal offers both

there is a pay now button and set up if you think no one will purchase more than one thing

there is also a shopping cart set up for paypal which will accumulate the buy this clicks, add sales tax when applicable and take payments.

The advantage of paypal or a similar site, IMO, is that the transaction does not hit your site so there is no requirements for credit card security, paypal handles all that.  Just leave the actual transaction receipt in the paypal account and only get an email from them telling you what was ordered, by who, and where to ship it if you are selling physical goods.

My recommendation is to never use the buy now button, always use the shopping cart set up, you never know when a customer will want more than one thing, or if you will offer more than one thing.