Travelstart Flap deception involves Black Friday

I booked a flight on Black Friday from Cape Town to Durban, based on the R200 discount offered on the Travelstart Flap app. After processing the payment I saw that the discount was not app****. I called the contact Centre and was advised that all the discount vouchers were already redeemed and the discount was no longer available. I find this completely misleading as the offer was still available on the app with no mention of it being expired or limited. I tried to cancel my booking on principal and was advised that because this was a Mango flight no cancellation was possible. I am extremely dissatisfied with Travelstart and their deceptive advertising