“Poppin tags” means removing the price tag of whatever you just bought.

In the context of this song, popping tags means removing the tags of a thrift store item in store so that it has to be repriced in hopes that a cheaper price will be assigned to the item.

($20 buys a lot of clothes because thrift shop prices are so low)

Additionally, the expression “pop a tag” parodies another phrase commonplace in hip-hop, to “pop a cap” (i.e.: to fire a bullet). It references Gangsta rap lyrics, which were heavy on urban violence and talk of “popping a cap” on someone the author didn’t like.