International Priority Mail to SD

I'm concerned reading the posts about the USPS. Yet they tell me that I can send a priority mail flat rate box of books taking 5 - 10 biz days to arrive in SD. I once sent a book first class and it arrived no problem.

However, I can't put a tracker on the priority mail package. Has anyone had any experience (positive I hope) using this service?

I have sent stuff through a courier in FL and it was very expensive. This box weighs 18 lbs - have plenty of time for to arrive - need it there by the 20th of October.

Any suggestions, advise, etc. greatly appreciated.

Thanks
Ro

Purolator is from Canada to the DR - according to their website. I need information from the US specifically if anyone has had experience with priority mail flat rate large box service. Under 20 lbs is about $50 and takes 10 biz days compared to the Fedex' and DHLs who are a couple of hundred dollars.