the answers below are true and all, but - they don't answer the question at hand, so.
he says grocery bag, because when people in (but not limited to) the hiphop community,
get girls they call it 'bagging em'/'bagging b*tches'. it's just another twisted way of saying getting girls.... like how when they call oral sex 'head' .... and then they started callin' it 'brain'... they're just trying to be clever haha. bye.
Its like saying I got her tight like my shoe strings. Hes speaking metaphorically. I dont know why he said grocery bag LOL. Who knows what those rappers be on sometimes.
you must be the 1,000,000th person to ask this question. it means nothing, young money is a bunch of 5-year olds that cant write rap so they add anything in that rhymes with the previous last word of the last line.