Hmmm, this is a really interesting question.

I've worked with the letter books for the Providence Island Company from the period reasonable extensively before, so I can say that letters at least got to the Caribbean Columbia with reasonable reliability. But that was also very much a private endeavour using their own ships for transit as I understand it, though the personal letters of those tangentially related to the company are referenced as being included & delivered too so...

Like u/Salamidick I'd be really interested to know if there was a more widespread "public" service and what the pricing would be like.

Edit: I'm an idiot who got the location of the wrong the colony wrong, eh it was 5+ years and another thesis ago its amazing I got the name right. Anyhoo, you can get the letters at the BL if you want them.

I woukd like to add to this, how much would that cost? How widespread would mail use have been?