This pissed me off as well, it's been the same since we switched to Macron for some reason. Don't know if it's something to do with Macron or whether Swag just conveniently used this as an opportunity to introduce yet another tax (I know what my money's on & it's got FA to do with Macron!). It always used to be a nice softener for me after buying the STs I could then get my daughter's shirt at a reasonable price.
For the discount you also only had to swipe/tap your ST, no code required.
You can't use your discount on any Macron stuff - that's half the f*cking store! More importantly, it's really the only stuff people actually want to buy! Basically, you can get your discount on a mug, pencil, beach towel or 'Run JRC' T-shirt, fantastic, I'll start the car now boot up the laptop now!