When adding a sunglass frame to stock, the VAT fields should be as follows:

The VAT category should be set to Fully VATable (This is so if it's sold through the till as plano, the correct VAT is applied).

The Disp VAT Category box should be ticked.

Also, the When Dispensed VAT field should have an option where the description matches the Sale Price (this is so when the sunglasses are dispensed with prescription lenses, the correct partially VATable amount is applied).