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But people will love to have the "Pay with Steem" button on our website! Or the "Pay with SBD" button! Right?

Yes I imagine they would. Not just pointing out that it is "Legal" to accept Crypto Payments in the USA and UK (two biggest territories for namecheap.com). It is just a matter of coding in a system to provide Memo Key when user clicks the Pay with Steem button and then monitoring your @steemhost account for the payment to be received on Steemit.com