I believe you will find your answer here on the IN state DOR web site
Sounds like you only pay your states tax rate not Indiana's rate (which appears to be higher than NC).
Here are a couple of quotes from the FAQ section.
"My customer lives in another state that has a sales tax rate lower than Indiana's 7% sales tax rate. Will the buyer be entitled to a refund for the difference in tax rates?
Motor vehicle dealers (retailers) should collect Indiana sales tax at the rate the buyer would pay in his/her home state. The Indiana dealer must complete and retain a Certificate of Gross Retail or Use Tax Paid on the Purchase of a Motor Vehicle For a Nonresident (ST-108NR) to document this sale. This form includes identifying vehicle information and an affidavit for the purchaser to sign certifying that the vehicle will be transported outside Indiana within 30 days after delivery, and will be titled or registered in another state or country. Although the Indiana dealer will charge sales tax at the other state’s current tax rate, the tax collected is Indiana’s sales tax and must be remitted to DOR."
"I am a RV and/or cargo trailer dealer with a customer who is a non-resident of Indiana. Must I collect Indiana sales tax (7%) if the customer will transport the purchase to his home state for title or registration?
Yes, nonresidents are subject to Indiana sales tax unless they register/title their purchase in a “reciprocal” state within 30 days of purchase. A state is reciprocal with Indiana if the other state allows a sales tax exemption to Indiana residents who purchase a recreational vehicle or cargo trailer in that state (assuming the purchase will be registered or titled in Indiana). No exemption exists for a customer who is registering or titling in a nonreciprocal state or any foreign country.
A purchaser claiming a sales tax exemption must complete an Affidavit of Exemption by a Nonresident of Indiana on the Purchase of a Recreational Vehicle or Cargo Trailer (ST-137RV). The selling dealer must keep this form at its place of business. A legible copy of the form should be sent to DOR for review. Tax must be charged for the nonreciprocal states and all foreign countries, as shown in the below chart.
As of July 1, 2020, when a nonresident intends to register the vehicle in a "nonreciprocal" state, the dealer must collect Indiana sales tax at the rate of the destination state where the vehicle will be registered. These nonreciprocal states are as follows:
*Cargo trailers only are taxable in these states. RVs are exempt."