QBO is not really suited for this even if you had QBO+ with inventory
Post the purchase price to an asset account you name purchases, that includes the printing price
for each shirt you sell do a journal entry (or several shirts at one time)
debit COGS and credit purchases for the total of $5 * number of shirts sold
you may have to create an account called COGS in essentials, I am not sure if it has that type of account but if not the account type can be created as an expense type account
