Usually, 袋 is an open container like a sack, and 包 is a closed container like a packet. But that does make this sentence weird, because ... I usually buy my sugar in closed containers, not by the sackful.
Amongst the two, 帮 cannot be used as a noun (help, aid), but 帮助 can.
Personally, I tend to use 帮 for "to help" (verb) the most, and 帮助 in more formal contexts where you would maybe used "to aid" (verb) in English. Of course, there are more nuances than this.