U.S. DOT guidance for delayed baggage here:

https://www.transportation.gov/lost-...%20liabilities

Taking into account that you will probably get use out of the purchased items well beyond the three days that your one bag was delayed, if you still think that reimbursement of only 60% of your expenses was unreasonable, you can consider filing an air consumer complaint with the DOT.

And, if you paid for the airfare and baggage charges with a credit card that provides supplemental delayed-baggage insurance, you can also consider filing a complaint with the card's insurer for the amount not reimbursed by Frontier.

Good luck!