I used to offer Bitcoin and Litecoin. I no longer do, because it was rarely used and a very bad deal for me in terms of various fees. Also, cryptocurrencies aren’t real money, they’re mostly used by gamblers, tax dodgers and separatist nutjobs who want to go live in the woods.
As of 2021 I offer direct credit card payments. This used to be possible via Paypal checkout (and still is), but you can skip that step.
Paypal is still available, your best bet if you don’t trust me and want a third party to complain to.
As of 2021 I also offer Express Checkout, so you can use Apple Pay or Google Pay on your mobile device.
I used to offer a bundle under the name ‘A Treatise on Christianity‘ which actually contained three of my books. It was to hide the name of your purchase on your credit card statement. It was only ever used once and so I no longer offer it.