Wow, an Amazon virgin! I'm like a whore compared to you
Anyway to answer your questions;
1. The USPS options from Stackry imply that the local bunch of idiots at the Post Office handle your parcel at this end. Use this method at your own peril. I recently collected a parcel from the Post Office that I bought in August last year.
2. The Fedex options from Stackry will ensure you get your parcel but it's expensive.
3. Your current Amazon order that is shipping to you directly will attract no additional costs.
4. In terms of when to use Forwarders or let the merchant ship directly, it's NEVER a clear cut option that you can rely on. You have to punch the numbers every time you want to ship something. Not all items on Amazon will be eligible for delivery to South Africa, especially if it's from a 3rd party seller. You then have to use a Forwarder like PBC or Stackry.
Even if Amazon will ship, sometimes it works out to be more expensive than the Forwarders but not always. You need to compare shipping costs every time.
Remember one thing, even if something is on sale at Amazon, they use the normal retail price to calculate your import duties and VAT so that deal may or may not be that great after all.
One advantage of using Forwarders, is that you get to complete the manifest in terms of invoice values. Use your imagination to work out what this means to you as a consumer paying VAT.
Without a doubt, if you have multiple parcels from various suppliers, the Forwarders will be cheaper to consolidate. If I have a bunch of stuff in my Amazon cart and they are all eligible for SA shipping then what I do is to move each one in and out of the cart, making as note of the total estimated shipping costs. This is because sometimes the additional shipping costs for something coming out the same warehouse is negligible not always, hence the cart shuffle.
One other thing. The PBC depot is in Florida and certain merchants charge Florida Tax so take this into account as sometimes it may be cheaper to ship to Stackry in New Hampshire.
Shout if you need more info.