Paypal used to be the easiest thing to integrate - especially because they would handle the entire backend server workflow, and you just get a ping from their back end when the thing is done. They also have the instant payment button that would accumulate payments and consolidate orders without you even having a backend.

If you're looking for something similar to the payment integration, Stripe is pretty similar in terms of development effort and can even run a form on your (static) web site. You still have to do a bit of processing on your end, but we're talking about a few hours of work tops.

We ran a subscription business for a few months without a real backend or a database (validating that there is a demand) using stripe and JotForm. Jotform is a very simple forms builder and server online that can handle a trivial product selection, collect custom information, execute a payment workflow and send the customer and you an e-mail at the end, optionally showing the customer a confirmation page. Yes, all this is doable by a good programmer in a day or two, but it costs next to nothing on JotForm and allowed us to play with the process until we got it right. Their site can run paypal and stripe (and a few other things) so in case paypal blocks you it's easy to switch to a different provider without customers noticing anything.

Stripe doesn't offer seller or buyer protection - which is another benefit of PayPal. I don't know anyone else who does that.

Does Google Wallet have a checkout service? If so, could be one alternative.

Otherwise, their is Braintree, Stripe, and more similar services.

EDIT: Seems like Braintree is owned by Paypal. So not really an alternative.

It depends on what countries/currencies you do business in and which types of transactions you need to support.

As per https://www.chargebee.com/partners.html businesses can choose Authorize.net, Braintree, Paypal, PIN, WorldPay, eWay etc.

Stripe.