You can use a money order. Money orders can be purchased at most banks, but usually the US post office, has them the cheapest. It will cost a bit extra to get a money order, but it’s usually around $1 for every $100. For example it you are going to buy something that cost $56.00, they will charge you $1, up to the $100 mark. If it were $101, it will cost $2, for the money order. So, the $56 purchase will cost $57.
Before you do this, check with the company or seller, to make sure they will accept a money order. Money orders are like cash, in that they are guaranteed on demand of payment.
They are like checks, but excepted more widely, because they can be cashed anywhere. And, there is no wait time, to be sure it will clear payment, as a check will.
The problem is, you will have to be sure they will take them, and you do have to go out to get one, and then, you must send it via snail mail. Once they receive it, it should be accepted as payment made.
I caustion to check they will take the money order because, it is an outmoded form, (like checks) these days, and this will only work in the US. There are Canadian money orders, and some international money orders too though.