What you have described is indeed the usual case.
Normally the point-to-point link between a customer's router and the provider's router is addressed out of a separate /30 address space. In addition to this /30 subnet the customer is also assigned another block of addresses that are routed by the provider to the external IP Address of the customer's router. If a customer requires more addresses in the future the customer just requests for them and the ISP routes them to the same external IP Address (one of the two /30 addresses)