Seems to be a glitch on the prices - listed correctly in the blurb but when you choose the option you want they are priced incorrectly 

They should be 33 - 50% off the on site prices of £6 for 1 wash - £30 for 5 washes but Groupon have mistakenly put £6 as the normal price for all options & then taken 33 - 50% off this price

so as per example below 

1 wash is £4
3 washes is £3.33 (should be £10)
5 washes £3 (should be £15)

Use code: HELLO15 for an extra 15% off making it £2.55 for 5 washes