What's the converse of "The straw that broke the camels back"?

Two idiomatic phrases would be "the cherry on top" and "icing on the cake".

Your question does't match your title. The straw that broke the camel's back matches what you describe, although I think the usage as the final straw, would sound a bit better. Possibly something along the lines of nail in my coffin or bowled me over.

Sorta the reverse, but also possibly a bit of the same would be "to boot" as in "and she can cook to boot". The reason why it might be both is that it is used in both senses: toss x in to boot and you've got a deal vs I'll toss x in to boot after the deal has been made.