To customers who wish to use the coffee machine: we will provide cups after you pay at the register
文章で案内したい場合、「Customers are asked to pay before using the coffee machine, and we'll give you cups.」は通じると思います。私だったら2文に分けると思いますけどね。
例えば、
We ask that customers pay before using the coffee machine. We will give you cups (at the register)
ご参考になれば幸いです。