Even if clearance items can be a headache for retailers, they still have to designate that such items are on clearance, and that all sales are final. At the time the OP bought the items, there was no indication that the items she purchased were on clearance, or were going to be on clearance. The OP was under the impression this was a regular item and purchased it with an understanding she could return it.
I work for our State's AG's office. I would definitely contact them. Even though the amount is minimal, the store was in clear violation of their refund policy. The receipt states she has 90 days to return the item. This being stated on the receipt makes it a legal and binding contract between the retailer and the consumer. Had the receipt not stated she had 90 days to return the item, then the retailer has every right not to issue a refund. Even though the retailer changed their refund policy, it was purchased before the date their refund policy was in effect and therefore the consumer is entitled to a refund. Especially if the retailer did not have their refund policy posted.
Imagine if 100 people are having the same issue as the OP, and they each spent only $20. While $20 may not seem like a lot to you and me, that's $2,000 in the retailers pocket.