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. These are handled via Stripe. I have disabled Paypal, because of their high fees. However, I can switch it back on for you if you really want that option. Just let me know. (If you find it activated amongst the payment options, feel free to use it. I may change my mind at some point.)
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.