I can tell you about my 1 and only experience receiving a package. Up front, I will point out that it did arrive so here is what happened:
I received a little card in the mail at my dpto to go to Serpost and pick up a package that arrived. This came on a Friday afternoon so on Monday I went to their office to retrieve my package. This happened 1.5 years ago, so I'll strain my memory a bit.
I walked into the office in Surco (I think it was) and right by the door there is a person you have to talk to and get some paper or something. The room wasn't that large, has several doors on the left and had two rows of about 5 or 6 chairs if I recall correctly. There were 3 other people sitting down when I got there. The girl told me to go to one of the doors and present the paper. That person told me to go to the next door and present some other paper and then told me to sit down and wait. So I waited well over an hour, probably 90 minutes, and then got called to the front window where the guy opens your package. Again, there were only 3 other customers in the building. So he opens the package and I had 2 baseball hats, a shirt, and a couple books that my dad had sent me. He proceeds to indicate the value of the items and his amounts were so DAMN high that I'm glad there was nothing else in there. Luckily the amount was under $100 as well it should be. Although the books have the prices on them, he was marking down triple the price which I pointed out to him. So after this little bit of fun, he gives me another piece of paper and tells me they will call my number when the package is ready. So they call my number 30 minutes later and the guy proceeds to tell me that I am late in picking up my package b/c I didn't come within the 15 days or whatever the time limit is. Of course the little piece of paper arrived late so I had to pay the stupid ~10 sol tax which I went to another one of the little doors to do and then back to the other door to get my package. I think I spent over 2 hours in there and there were very few people. It is a big freaking joke.
After this ONE experience, I've never received a package nor ordered anything again and am quite happy not going to SERPOST. I did lodge a complaint as well. I'd rather eat cebiche de cuy than go to the post office here.