Having just waited over 2 weeks for tyres to arrive from the US to Ecuador, it´s a good idea to ship them up front (although for 7 weeks I wonder whether it is worth fitting them up front ? are your old tyres that rough or wont last 7 weeks ? so why not save the hassle & fit them now & save shipping
Anyway, Ecuador postal services for receipt are very slow & it´s murdurous to get info as to progess. I would expect the rest of S.A. to be similar.
I would contact the local communities for where you expect to need to change the tyres & see if someone will let you send the tyres to them, and then send via courier. (that´s what I will do in future)
Note that there´s an interesting thread re a well known courier on this forum, from someone who swears by them ... but not in a good way >
when the road is straight & true, I romp ahead
when it twists & turns, I make the best I can - Lao Tsu