What should I do if a good customer wants a reward for how much they spend?
For online or in-storeproduct sales: A simple rebate that has proven to be very effective is free shipping with a minimum order (say $50). Places like L& L Bean and Kohl's switched up their holiday sales incentives by offering a $10 gift card with any purchase of $25 or more, $50 or more, etc. Bakers dozen (getting 13 of something for payment of 12) is another idea.
For service sales: Many creative service-focused entrepreneurs "super-size" their sales by tagging on a FREE additional service with certain purchase amounts or packages (a marketing communications firm offers free press release pitching services (worth $245) along with a purchase of one advanced press release package ($550). An interior designer waives the In-house assessment/planning fee ($395) with any design purchase of $500 and up, etc.
Additional ideas that can be fitted to suit product or service sales are: Giving away something (worth XX amount of dollars) with larger sales (a variation of the furniture industry's famous "loss leader" approach) which could be apparel, books or whatever has tangible value to your client (a free report, a discount on a future sale within a deadline period of time, etc).
Hope this info has helped--feel free to post me a note directly if you have any other questions.
Good luck with all your endeavors!