A rebate is an incentive that you offer as a supplier, or receive as a purchaser in many different circumstances. If you receive or provide a rebate, you may need to adjust the amount of GST you've claimed or paid, or treat the rebate as a separate sale, depending on the circumstances.

Does cashback include GST?

As advised, most stores will only pay Cashback on the purchase price excluding GST and shipping, delivery and any other such costs. This mostly explains why you can have a lower purchase amount reported and consequently receive a lower Cashback amount.

Do rebates include tax?

Generally speaking, the IRS considers transaction-related points or rewards as rebates, and not as taxable income. Think of the rebate as a discount you'll receive on your purchase later.