According to the Ohio department of revenue, if the price of the sale changes, you make the adjustment when the product is delivered.
Take a car sale for example. (This is the most obvious example). A person purchases a car for delivery on a particular date. The sales contract is written up assessing sales tax for the purchase as contracted, regardless of when the car is paid for. A deposit toward the car was made, and the balance would be paid when the loan is approved. Now comes the customer to pick up the vehicle later, and since the original contract, has added a stereo, undercoating, etc. The dealer then re-invoices the car showing the original contract price and sales tax, then he lists the additional add-ons; or he invoices the add on separately. Whichever method is used, the add-ons or additional pricing is added, with its concurrent sales tax.
It would be done similarly to your situation. Through the course of creating the product for your customers, they make changes. Those changes will add to the price. When you deliver the product, you would include an invoice that itemizes the deposit, the original sales tax, and then the additional price for modifications and the additional sales tax listed separately.
That line would be labeled on the invoice as words to the effect;
Art Project Piece 1200.00
Sales tax @ 7% 84.00
Balance Due: 684.00
Changes and modifications 300 dollars
Sales Tax @ 7% 21
Total additional charges 321
Final balance due: 1005.00