This is an age old question: a customer asks: "I've got 150 tickets but I want the 200 ticket item. Can I pay the difference?"

My answer is: "sorry, we have no way to do that, but our tickets never expire so you can save them for your next visit." I might even add: "Here's ten tickets to add to your total." Its better yet if I can promote my FEC's "Super Savers" or other "VIP" club where the customer can receive elite status at a certain annual and/or lifetime ticket total.

I always advise not to sell items for three main reasons:
1) Redemption is a fantasy world of fun and excitement with its own currency-tickets. By letting people buy, the experience is diminished.
2) We generally set up our clients to run at a 15% cost of sales or less. In the retail world thats a markup of 6.7; Toys in the retail world average a markup of 3. Why trade a markup of 6.7 for a markup of 3?
3) A good redemption program is designed to promote the saving of tickets. Redemption then becomes a loyalty program for the facility. If people are saving your tickets then, when they and their family or friends decide to go out and bowl or play games, they'll pick your FEC since they are building your tickets.

Allowing them to buy in goes against all of these goals.

At the same time we recommend a liberal policy for situations like this: a little kid has 20 tickets, Dad is standing over him (often impatiently cause Dad's ready to go) and the kid wants the 25 ticket item. Dad asks- can I pay the difference? We recommend that the redemption pro say "that won't be necessary, we're glad you came today so please let us make up the difference." Using the economics we set up for our clients, those 5 tickets cost you 2.5 cents and you've probably made a customer for life-plus, its taken care of in the rounding of your price points.

Obviously you have to have well trained employees to do this judiciously.