Do I need N confirmations if I'm only worried about the transfer getting invalidated by a different block? Say I ask Bob a lot of bitcoins in exchange for an expensive product. Bob sends it to me and I wait 1 confirmation to send him the product, because I trust Bob. I am only worried about the transaction being invalidated by a block that doesn't contain it getting solved. How can I determine a "safe" number of confirmations as to make that risk negligible? When I read about the "6 confirmations" thing it's always about preventing double-spending, and some even say that if you trust the other part you don't even need confirmations. Wouldn't that (zero confirmations) put the receiver in great risk of having the transfer invalidated?
A transaction can only be invalidated by an attempt to double-spend. That is, the same transaction can be in block X or block X+1 or block X+n as long as there is no double-spend associated with it.

If you want to make sure that a transaction goes into a block soon-ish, try to add a fee that's sufficient to make a miner want to add it to a block. In that case, you only need a couple confirmations as being an orphan block after that is exceedingly unlikely.