Withholding tax and Certificates in Puerto Rice

We are implementing in PR and have discovered that they have a 7% withholding tax for local vendors.  The tax only starts deducting after we've already spent $1500 with the vendor.  We have accomplished this by creating a code with Amount Basis set to "Gross Amount" and in the tax rates section, for 0-1500 we have 0 rate and for 1501 onwards we have 7%.  This is all working fine.  The question is around the certificates.  For some vendors we also have certificates that result in a 3% withholding tax as opposed to 7%. This means that from 0-$1500 we