The promotions and coupons they offer are usually meant for credit cards users only so even If you manage to pay using bitcoin (or any other payment method), you won't get any discounts, just use Namecheap as suggested above.
Thank you for the information. I did not know that the coupons are only valid in connection with credit cards. That's a pity. That was my only hope.
I'm thinking about switching to Namecheap. Does anyone know if the change or the transfer of the domains is associated with costs?