The official rule on the show was that you could put any amount of your winnings from a round "On Account" at any time while you were "shopping" -- in fact, I recall one episode in which a contestant put part of her winnings "On Account" before buying anything else during her "shopping spree."
Usually, though, a player would "shop" until he or she ran his/her bankroll down to less than the price of the lowest-priced prize that was available, at which point that player would be given the option of putting whatever amount was left "On Account" or taking a merchandise gift certificate of that amount from a participating sponsor of the show (more on that later) Indeed, they could even split that amount, with some of it going "On Account" and the rest on a gift certificate.
Longtime fans of the show, though, remember that using the "On Account" option carried risks: If the player won another round on the program, the "On Account" money would be added back onto the player's winnings from that round and become available for more "prize shopping" after that subsequent win, but if a player did not win another round on the program, that money would be lost (cash was never awarded as a prize on the show except during the "Bonus Round" and during the time when they had the "Cash Jackpot" as an extra wheel prize in Round 3 on the daytime version). Also, any "On Account" money a player had from a previous round would be lost if that player hit "Bankrupt" on the wheel during later play.
Michael Brandenburg
(But you know something? When Chuck Woolery did the show in the early days, he'd drive me nuts trying to figure out what a "tift cergificate" was? Only when he took to talking a little slower that I fount out that it was supposed to be a "Tiffany gift certificate" from that very fancy jewelry store of the same name!)