Twice I shipped merchandise to Pr and did not pay any taxes. Most items were antique, used furniture and art stuff. My understanding taxes are on new items like electronics possible resale items. I will be shipping my last container including my art/business materials. This will be later in the year and I will post the results
The only item shipping where you should need to pay taxes is autos. You can put in the VIN on the Hacienda website to get an estimate of the tax that will be due (it can be $10k+ depending on the make/model).
As mentioned above, if you're importing merchandise for resale you need to pre-pay the IVU electronically before the container will be released. Items that are personal and not for resale should be exempt.