Post Office charges on items shipped to Colombia

Since I moved to Medelllin last June, I have been using BoxTCC in Miami (a freight forwarded run by Colombians and ships only to Colombia) -  boxtcc.com.co/en

When you sign up (no fee), you get a "Box #" you use in the shipping address for all the things you want shipped to you in Colombia. Their fees for processing your shipments are low, they help you set the value of your shipments (you can do online easily), and they get the lowest Colombian customs duties (I think 10% for shipments under $100, and little or -0- for medications), and (I think) because of the shipping volume they do to Colombia and that they are run by Colombians.....I have never had Customs question my stated value of my shipments. And they deliver tight to your door.

My Customs duties + TCC processing ans shipping fees, depending on the content and value, run between 10% and 30% of the content value TOTAL.

I recently had 4 small shipments arrive at TCC. On their website, I asked them to consolidate the 4 into one shipment, one carton. BUT they are smart to know that one of the 4 shipments was vitamins (which have very low Customs duties), while the other food and kitchen items would be subject to higher Customs duties. So they made two shipments - one with the vitamins and the other with the other 3 small cartons into one carton. Saved me money!

Best to use a credit card and pay online when they send you an e-mail saying a shipment has arrived in Miami for you. But you can also pay C.O.D. but you need cash to pay the delivery man. If you pay online with a CC, best to use a CC which you have advised you are living in Colombia and using the card there....as online CC transactions are processed in Colombia, not Miami, USA. Otherwise they can e-mail you a CC payment form to complete, scan and send back to them - which is processed in Miami and does not need to be a CC you are using IN Colombia.

If you have any questions or later problems with your shipments (I have had none), you can call -  305-499-9938 , then enter extension 8000    to speak with Marta or Shyra who speak good English.

To answer the OP, the taxes are not set by the Post Office, but by Colombian Customs. The taxes are often based on Customs officer own evaluation. It can be negociated if you can prove the value of the taxed items are lower than their evaluation. A proof would be an original invoice. No need to mention it will require time and effort.  First step would be to inquire about the import taxes applied for each item. Import taxes are publicly available, but honnestly not easy to decipher. 
Frankly, unless we're talking about thousands of dollars, I would just pay.