I worked there part-time about 6-7 years ago. It pay min.wage + some items you get commison or a promotion on. Most of the items that have P.MS (promotion) are GNC product; however, times may have changed and the pay system may be different.
The commission usually is on GNC brand products. You get what is called a "PM". When I worked at GNC, super Tuesday was our big PM day. Our store would get busy on Super Tuesday. If another employee was busy helping another customer and someone was at the register and ringing another customer up, the person at the register would give some of the PMs to the person helping the customers.
I work there. It's 5.50 plus COMM.... Usually commision for a day is between 20-40 bucks... On gold Card week it ranges from 40-100 bucks for me... Most of the time with the Comm added in I make 10-14 an hour.