Always read an artists ToS (where available) COMPLETELY before commissioning them.

These are the terms you're agreeing to and are legally bound by when an artist accepts your commission.

And if it says payment is due within 24 hours of the invoice being sent, you need to pay within that 24 hours unless something has been worked out BEFORE you placed the order.

If you don't have money to pay right then and there (for artists who require payment upfront at least) then don't commission them unless you've talked to them first and they've accepted you paying at a later date!