When you assign prices to fields, a calculation rule is created automatically. This rule multiplies the field (actually, its assigned price) with 1 and can be found in the Summary and Discounts section. When the calculations don’t work for your form, this is the first place where you should check that the settings were made correctly. In case the multiplication by 1 is not there, set it up manually: Settings → Payments → Summary and Discounts → (e.g.) T-shirt x 1.
For advanced calculations, use a Number field with the numericplus validation and add as a formula in the Assign Values or Formulas to Fields section.