billing and vouchers

I bought twice already on Zando online. The first time i bought a watch and paid cash upon deliver and the second time i paid via EFT because it had to be delivered to someone as a gift. The watch broke - the strap of my first order and i requested for it to be collected and then you get a voucher - but today now i want to use the credit note now they tell me i already used the voucher - which was not the case - then they ask me was the watch collected?? shouldn't they know this and now they tell me it was never scanned in at the warehouse? how can they have so many excuses and now know what is going on - maybe its really not worth it to save time and buy online if this is the case - its unacceptable.