I'm planning to have the characters of my campaign visit various countries all over Golarion for the sake of variety of locations and themes. That said, how could I achieve this within the boundaries of the setting and rules?
I'm guessing most travel happens by caravans/horse on land and ships on sea, but that can mean weeks of travel to go from country to country. Teleportation or portals sounds like the most practical and fastest way to do it, but then it becomes an issue of means, as I don't suppose these things come cheap, otherwise we would have airport-als all over.
Is there something in-between? If not, how could I justify such means being available to low-level (3-4 at the moment) characters?
Note: while it's always possible to narratively skip travel time and this can be a solution at times, I'm still looking for an actual fast-travel medium to lower the in-character time investment. It's easier to justify characters doing a "quick" sidetrip to a far-away country, less so if it's take 6 months of travel. It also opens up time-sensitive adventures across multiple locations, though this is isn't my top priority.
Best Answer
If you're constraining yourself to Golarion canon
I don't think you have many options, other than giving them a magic item that lets them travel fast. The flying carpet that mxyzplk suggests is flavorful and classic, but if you aren't comfortable with them flying you can create something of your own, like a chariot figurine that can act as the Phantom Chariot spell for 10 hours / day.
Otherwise
This is your Golarion. If you want it to have fast travel options available to low level characters, go for it. There are many ways to do so. To decide how you want it to work in your world you'll want to answer a few questions for yourself: