Flipside

Now I am translating the handout for financial accounting students. This is one of teaching assistant's duties.
..╮(﹋﹏﹌)╭.. What's my point is that in the paper there's a sentence "Flipside of purchase returns and allowances." I don't know what's the word "flipside" meaning honestly.
Therefore, I went for google and found out this song "Flipside" by The click five.

It's been a long, long time since everything was cool
I shoulda seen it coming but I guess I'm not the only fool
There's something growin' on the outside
Too much missin' on the inside
Should I waste my time and let you lead me on and on and on and on?

Waiting for the day when I'm complete
Without you I'm doing what I can to let you be
Making sure there's nothing showing on the outside
Something's dying on the inside
I'm still broken but I'm free
I'll see you on the flipside

I've got a bruised up heart
But I'm still hangin' out
I should take it easy but I'm still gonna get around
There's something growing on the outside
Too much missing on the inside
Should I waste more time when everything is done and done and overdone?

Waiting for the day when I'm complete
Without you I'm doing what I can to let you be
Making sure there's
Nothing showing on the outside
Something's dying on the inside
I'm still broken but I'm free
I'll see you on the flipside

I'll see you on the flipside
Nothin' showing on the outside
Something's dying on the inside
Waiting for the day when I'm complete
Doing what I can to let you be
Nothing showing on the outside
Something's dying on the inside
I'm still broken but I'm free
I'll see you on the flipside

Somehow, I like this song a lot.
Even I still do not know what does Filpside mean. :P