"A consumer buys a printer for her computer and receives a money-back offer. All she has to do is fill out a form and mail it to the company with a copy of the receipt from her printer purchase. Then the printer company will mail her a check for $20. What sales promotion technique is this an example of?"

Rebate or cash back programs are campaigns in which the consumer receives part of the amount spent on a purchase in liquid money or in free product tests. They are a good way to loyalty to the customer, since on the one hand savings comes into play, but they are also very interesting for companies, since they increase sales and are cheaper than direct discounts.

Mainly they work because they raise savings in the consumer's pocket, since after making a purchase, you can receive a percentage of refund in your pocket or receive the amount to be reimbursed in new products. There are mainly two types of cashback:

Simple refund: once the purchase is made, the consumer can choose to receive a percentage of the total price of the product promoted, say 15% for example. You can receive it either in the store or directly in your bank account.
Full refund: equivalent to receiving a free sample. The consumer sends the purchase ticket to receive the entire amount of the value of the promotional product or a second free unit. These promotional actions are very effective and call the consumer a lot.