No service from Crocs online shop

I ordered a pair of boots online on 25 July, they were delivered a few days later.When I tried them on, I could not get them on my feet. I wear a size 6 but the top part of the boot is so hard that the bridge of my foot could not get into the boot at all. I went online immediately where I was assisted by an online customer service person who after suggesting alternatives which I chose not to take, said they would arrange to collect the shoes and give me a voucher as I couldn’t get a refund due to it being a sale item. A week later, still no collection. I sent an email to customer service on 2 and again on 6 August. No reply. I phoned yesterday (8 Aug) and after stating my problem and lack of response the lady answered “ok perfect”!? She then said I would get a voucher which then could only be used for online purchase and not for in shop purchase. I said that I felt this was not a satisfactory arrangement as I was returning the boots due to a fault, not because I had changed my mind. I feel that I should at least be able to use the voucher in store if I am not able to get a refund. She promised to take it up with her manager and email me before 4pm yesterday ( she clearly stated that they close at 4) No email or any contact was made yet again. What must I do to get a response?