Wiktionary
cap it all off
vb. 1 (context idiomatic English) To finish or complete something. 2 (context idiomatic English) To surpass or outdo something
Usage examples of "cap it all off".
And to cap it all off, he was pretty sure something was jammed up his ass.
To cap it all off, Ivo would probably be on roller skates, which meant he could move considerably faster than the police.
And just to cap it all off, what had Major Sorensen said about Sellars?
He meant to cap it all off with a pint of Guinness and some local color.