I dont't think that there is anything illegal about paying for services rendered at the time of delivery. Nor is there anything wrong with pre or post paying.

As mentioned below a reason to have a gap between deliver and payment is typically

1) to manage cashflow 2) to inspect good and services 3) the buyer is the middle man or the service is part of a larger project that needs to be fulfilled before the person gets paid. (This can be part of 1.) 4) To use the cash, leverage on something else and wait to pay vendors - this period is called the float. It is essentially an interest free loan that the vendor makes to the supplier.

You could use an escrow service for payments by essentially moving the money from an active cash account to a holding account or 3rd party holding account that releases to the vendor.

But in the long run, hold onto cash until you have inspected and approved and then pay immediately.

My typical terms are 10 days and I get paid in 17.