A good idea but it will just be best if he pays the shop in their legal tender direct except if they do not accept fiat payment which is rare, spending in fiat will be better than going in circle of buying this Altcoins
Based on preference, it seems OP just wants to make payments in cryptocurrency, which is not necessary if the shop accepts fiat payments. Perhaps if I happen to be in OPs situation and I have two options of payment, even if Bitcoin is my most preferred means of payment, I will rather consider paying with fiat since the transaction fee for Bitcoin is too high.