The Outnet review: Returns and refunds

I made a huge order on the OUTNET worth about 3K and I ended up keeping just 2 items so the total refund I'm expecting is 2.5K.

I had sent the box back with the 3 copies of the invoice outside the box in a plastic slip ( the usual ones for shipping documentation) I sent this back on September 2nd and on the 8th I checked the tracking and saw that UPS was saying there was a commercial invoice missing. I called UPS and they said the retailer had to provide the information. I called the outnet and made them aware and they said they would contact UPS. I guess this wasn't done because UPS made 3 attempts to delivery the package back to me but I was already on holiday so I kept emailing and calling OUTNET and they kept saying " we have forwarded this to the corresponding team" on September 14th I called UPS and they said the package was taken to warehouse as it was classified as abandoned. I called OUTNET again and they said they would be in touch with UPS to open an investigation. By September 28th I called UPS and they said no investigation was opened so I had to open one myself. I keep calling and emailing and they keep saying they will contact me when they have a resolution or update. Also on their website says to attach all 3 copies of the invoice outside but UPS told me I should always put one copy inside the box for when they open the box to check in customs. So the OUTNET literally has the wrong information on their website.

Can anyone advise on what to do. Would my bank help me in this case? or should I take them to small claims court? It quite a large amount!