As a buyer, you can write anything you want (aside from anything illegal) into a contract, i.e. to withhold the last payment on a item until you check off the final product specs, but that won't make a difference unless the manufacturer agrees to the contract.
A contract that is agreed to (written and agreed to (typically signed), or verbal and verbally agreed to) is legally binding; a contract not agreed to is little more than a wishlist.
If this involves, as you say, online purchases or when the customer is remote and working through an agent, a contract can still legally binding, but the contract may be more complex. You may need to draft more stipulations into the contract with all involved, (agent and manufacturer); but again, those mean nothing if the contract is not agreed to.
See Contracts - Legal Information Institute for an outline of the steps of making a contract:
An agreement between private parties creating mutual obligations enforceable by law. The basic elements required for the agreement to
be a legally enforceable contract are: mutual assent, expressed by a
valid offer and acceptance; adequate consideration; capacity; and
The enforcement of breaking a contract is a completely different situation. You may draft arbitration arbitration (Legal Information Institute) into the contract; or may go to civil court.