Order Form Calculation

Hello-

I am trying to create a form for a team uniform order. There is a standard price that calculates in the total field based on the quanity ordered. But, if someone wants to personalize the item I would like there to be a different price.

I was using a hidden text box that would populate if a checkbox was selected, but the total field was not consistently adding up.

Has anyone done something like this with success?

Thanks!