Space Raiders (13g pack) are only 3 syns per packet.
The substitutes, even though made with free food (potatoes), still have to be synned what with them being a tweak (as flashharry mentioned). But if you didn't, I personally think that's more dangerous as you could be consuming so much more calories than if you just had 15 syns worth of crisps if you say you could just keep eating and eating.
Hope I don't sound patronising or something! But it's a habit you'd be better off working towards stopping. This comes from someone who before Slimming World, ate around 5 packets of crisps a day on top of a lot of other junk, aha. Now, I rarely even use them within my syns. I pretty much stopped straight away, and oddly enough didn't crave them. I knew I could have them within my syns if I really wanted, I just didn't
But if this is hard, maybe work towards cutting down slowly? If you didn't use any other syns that day, you could even have 5 packets of Space Raiders a day! I personally don't think eating that much helps to kick the habit, but it's a good starting point. But if you did want to use 15 syns a day on crisps, it's within Slimming World's rules, I'm just looking at the bigger picture
