I'm a bit surprisd that in all these replies, nobody has mentioned,... DO NOT PAY FULL PRICE UP FRONT FOR ANYTHING!
Don't even pay a large deposit.
Pay 10% at the most, unless you have ordered something special that the shop doesn't normally stock and will have no hope of selling it if you change your mind.
It's fair to pay full price on delivery. You've got the goods, they've got the money.
When you pay full price on order, once you have coughed up the money the shop has absolutely no incentive to rush your order through.
It has already got your money so from that point on, you have lost all your leverage to get the item/s you paid for.
In fact their incentive is to not hurry with the order because so far the sale has cost them nothing. The entire sale price which they got off you up front is so far 100% profit.
Their piority goes towards filling new orders that aren't yet paid for so they can get in more cash, so if you've already paid, that won't be yours.
The same applies when you pay a large deposit, say 30% or more.
If their total profit is say 30% on the completed sale then when you pay that ammount, they have already got all they were going to make on the deal, so from that point on, the rest of the transaction is just work for nothing.

That's hardly an incentive to hurry your order up.

Reputable shops generally have at least 30 days to pay their suppliers after the goods are delivered to them so there is no reason they should be demanding money off you before they have paid anything to anyone.
If their suppliers ask them for money up front then it's only because they have a record of slow payment and are therefore considered a risky proposition.
If you pay up front then you are taking on the risk that their suppliers considered was unacceptable.
Often it works but also often, it doesn't.

If the operation is so doubtful that it needs your money up front then just tell them that if you can't pay cash on delivery then you will look elsewhere.
Be very polite but firm.
They almost always accept this but if they don't, you're better off going somewhere else.