Tax exemption on Opera is not a process that can be arranged during the reservation process, it is a process can be arranged during the guest's billing.
1- View folio on the account that will be applied an exemption and determine the tax amount;
2- Then, with the related exemption codes below, enter the 8% and 18% sectors separately as negative. 
3- Before the billing, write the EXEMPTION number on the related reservation. (The number on the exemption document/card that your guest will declare.)
The box where you will write this number is usually placed on the Reservation More Fields section;
4- Then make out the invoice, on the invoice there will not be tax numbers anymore and the number that you wrote on the EXEMPT no field will appear automatically on your paper invoice/e-archive invoice or e-invoice.
