Many people when initially proposing a pricing strategy will consider the following aspects.
A: The cost of the blank product
B: The cost to decorate the product
C: The price the customer will pay
These three things are the main aspects of selling decorated apparel.
However, you need to make sure you're considering these additional aspects too.
D: Production Time
E: Overheads
F: Shipping Costs
Don't forget to pay yourself! Consider the time you're spending completing that order. From customer communications through to the physical heat printing, your time needs to be factored into the price of the product.
Don't forget to pay yourself!
Overheads are another factor that's easy to overlook, but complie over time. You don't want to be taking money out of your profits to pay things like rent on your workshop or electric bills to operate your heat press (even when working from your home). Factor these costs into your pricing model.
The final cost you may want to consider working into the price of the garments is the delivery fee. It may cost you to have garments and decorations materials sent to you before you even look at shipping costs to the customer. Work those charges into the product cost so you don't end up with multiple delivery fees taking from your profit.