would expect to pay 50% for a custom bike up-front, and balance on collection
this way its not legally registered as a “deposit” which is in essence, refundable (meaning the shop takes the hit if you walk..)
50% would be part-payment, and not refundable unless the shop failed to deliver the goods in the allotted time frame (they have then broken the sales contract with you)
50% payment would protect the shop for the risk of custom building a bike as you have then entered into a binding contract, and also cover the “cost” of many of the parts required for the custom build, so everything received in the balance is their profit / time
a production (stock) bike is a different matter because its just one of many exactly the same…would not expect to pay more than 10% deposit to “hold” a bike some weeks out from being available
and pay with your credit card, as others have suggested.