Think about how to consider underpayments

[sisou]

Can underpayments be considered and only a rest amount requested from the customer? Can those two paymens then be accumulated to fulfill the order payment?

Does this work and make sense for HTLCs in Phase 2?

An idea here was to look at an address balance instead of incoming transactions to determine if an order is paid, but that would fail if an address is re-used without being cleared.