Just a suggestion. Namecheap is having .xyz promotion (ending soon) and it cost only $1 for a .xyz domain.

Why dont you offer that to your customer? 0.01 BTC is valued at 3 USD. If I were your customer, I will ask you to register a paid top-level domain for me - .ga/.ml/.cf is free top-level domain right?

Although the .xyz offer is ending soon but after a year, if you customer make a profit, I dont think they mind paying more for a domain.