SERP packages.

Has anyone ever placed an order for this type of service only to see your ranking plummet within days of ordering?

that's a loaded question bro. there could be 1 million reasons your site drops in ranking (panda update, G-slapped, ETC). If you're link structure looks natural and you're not purchasing more blog comments when all you have is blog comments, you should be good. The threads in the BST have to get approved first, and you have to incorporate it into your overall strategy. hope that helps.