First post here so apologies if im in the wrong section.
I bought 2 credits to tune my 03 silverado yesterday and wanted to make some adjustments when i went to write my new adjustments in it asks me to buy 2 more credits or 6 credits to unlock unlimited to my specific year.
My question is, since I've already used 2 credits on my first write do I only need to buy 4 more to unlock unlimited or do i have to buy all 6 at once?
It sounds like you've already licensed your vehicle, you should figure out why it's asking for more. You only need to license your vehicle once and can then flash it unlimited times.
You get partial credit, look at the licensing window in the application under Help -> License Info. It's 75% credit towards the unlimited licenses, i.e. if you wanted to unlock all 03 Silverados it'd be 5 more credits. No partial credits are used towards the unlimited, so you get credit for using 1 of the 2 original credits. If you had licensed 2 03 Silverados, you'd get 3 credits towards the unlimited make/model/year as that's 75%.
https://www.hptuners.com/help/ has the info available online as well. Look for the Credit FAQ section, it has an detailed walk through.