Wow Momo — Charging 18% gst and misbehaviour

Recently brought food from wow momo Bangur outlet in Kolkata. As per the govt the gst should be 5%. Now when first asked about the bill the money rounded up to 183. However when the bill was going to paid at the counter the bill generated showed 245. I dont think this is legal for there seems to be dispute amongst the staff as to what is the price. Moreover the staff behaved pretty badly and did not show any documentation regarding it. They said about something that food cooked at the outlet is being charged 5% and the food being brought from factory will be charged 18%. As per the govt there isnt any rule in accordance with taking various gst for various items. This just seems vague and illegal. Please look into the matter or else we have to approach the public welfare council regarding the matter. This is just robbing people. Have been buying from WOW momo since quite long however was pretty much disappointed with the behaviour. Now i dont think a resturant can start taking vague gst suddenly. Look into the matter please!!