Bad customer service cause by school promo

Maybe I’m the dumb one so I went to Protea Glen Branch to refund a shoe coz the size was wrong and I wanted I wanted a different brand which they didn’t have a size 2 on .:

The Promo said buy school wear ,accessories and stationary excluding character for R650 which they they bought a trouser ,a shirt ,socks &shoes that resulted in us saving the R150 now when I go there and I want another size and they don’t have it and school are re-opening tomorrow I’m told they will deduct the R150 on the school shoes coz that is were I got the promo maybe my math and reading is not make sure doesn’t the promo apply to all the school items

I don’t mind them deducting but can you make it clear how is the whole R150 deducted on 1 item when all items have a star to indicate promo

And the attitude I got for asking the explanation was not even making senses I left there with my shoes still not understanding and bored by the attitude and unnecessary fighting all eyes on me like I was an idiot

When you do promos please elaborate further for us incase we wanna exchange or refund things on promo items please coz the sales ladies makes us dumb and stupid with simple plan English