Again, I wasn't sure if the info was dated. If there is customs there and I believe that is true, then you (vessel captain) would not have to pay to transport customs officers to SP. There are always many issues with clearing in. Every country is different and I've found that some ports are different in the same country.
A lot of times there where overtime issues for the customs people, sometimes (quite often) customs would act as immigration, sometimes not. We always maintained radio nets with other vessels to get the up to date information for the port to be entered because it changes frequently. I'm not an airplane pilot so I know little of clearing in by plane although I've cleared vessels by hiking to the airport on occasion. If I was a pilot who wanted to clear a plane in at SP I would contact them ahead of time to see if this is possible.
Clearing customs was usually not a hassel and most customs people where pleasant if you where pleasant and reasonable with them. Some countries, not Belize, have a very poor reputation when it comes to clearing in by boat at least. Dominican Republic is one which comes to mind.
I have many amusing stories of clearing customs and I can usually be persuaded to tell them over a few rums.
Somewhere on a beach in Belize