If you aren’t going to be able to pay your bill by the due date, just let us know. We try to be as flexible as possible, and can almost always delay the due date of the payment for a while.
It’s far better to let us know about this rather than letting the invoice pass it’s expiry. Domains expire on the due date and some (for example .eu domains) add a redemption charge in the hundreds if the domain expires. If you let us know we can just advance you the renewal and we’ll avoid all this mess.