is it possible to revise the invoice and add to it the cost of a courier to the destination? I.e. in the postage cost section?

It may make the buyer think twice before paying again, and if they do, all well and good, hand it over to the courier and you have done the best you can?

apart from that, I'd ring eBay and Pay Pal. (I'd actually do this first)

{plus best check if you are even allowed to send plants and dirt to SA - they might have restrictions about what can be taken across the border)

Good Luck with it.