Paying with bitcoin on Namecheap works through adding credit. So you cant go through the regular flow of purchasing a domain.
One way of doing, which I like is:
- While on a Mullvad VPN connection which is paid with bitcoin, create a free proton mail account.
- Add credit to the proton account using bitcoin.
- With that credit, purchase the proton subscription that includes simplelogin.
- Use simplelogin to create an email alias
- Create a namecheap account with that alias
- add credit to namecheap using bitcoin
- purchase domains with that namecheap credit