Postal service costs

Same as in the states for USPS.
UPS is another issue, they seem to consider PR a foreign country so the charges are more that the US.
Amazon ships most items to PR, but other companies do not. Seems to be related to 2 areas:
a) Taxes, some of these companies have not done the paperwork
b) Bulk / Weight, some items are not worth shipping by boat especially when it takes several weeks to make the trip.

A lot of US stores are in PR, some will special order stuff and you can pick them up at the store, or they have their own subset of items that they normally carry for you to select from.

As to timing, add a couple of days extra for packages, but most times the same as US. Wife and I buy a lot of stuff from Amazon and have it delivered to the Post Office since our physical address is not accurate (no real road name or number and no house number). If your situation is like mine, Post office can give you their physical address to which you add your PO BOX number (Just the number like #123) and UPS and others will drop the packages there, they will put a slip in your box and you go to the counter to retrieve the package.

Everything with USPS is
the same as on the mainland.

Thanks Guy's! I was hoping that was so.