Charging for native files

Polo Shirt designs could mean almost anything. 

• Did you create custom pattern blocks or slopers that were used to cut & sew the garments?
• Did you create a brand label or insignia that was sewn on the shirts (Izod Lacoste alligator)? 
• Or did you design custom artwork that was screened or sublimated onto prefab shirts?

In my world, when a client commissions me to custom design something, they receive the artwork. That's what they're paying for.  I usually provide them with a hard print in multiple sizes as well as high quality PDF.  Clients may use the artwork anyway they see fit.

The time to negotiate prices is up front and have them sign a written agreement.  Wet or digital signatures are legally binding for BOTH sides. 

It's always a tricky one.

What was in your original contract / commission?
Did you assign copyright? Yes, still doesn;t mean you give them the works, just the right to use them.

Do you want to work for them again? Yes: think carefully about the multiplier. No: go for it