Refusing to issue a full refund

I've been a customer for well over 5 years and this experience has left such a bad taste. I had made a big-ish order and paid for majority of it in conjunction with some funds from my Superbalist wallet which were so small i struggled to get anything with them for months so I finally had a big enough order that they could be swallowed by. I returned one item from the order which cost less than the amount I paid from my card and they decide to refund the wallet in full and then the rest to my bank account. After emailing them and having a useless back and forth with an agent, they refuse to give the full cash refund because their "systems are automated" and so they pay themselves first. As if systems aren't made and managed by people. I returned the item for size reasons so now I can't even get it elsewhere because they're greedy. And I had read the Ts+Cs which spoke about refunds for vouchers and promotions but funds in my wallet are NEITHER of that because they're my money from a previous order. And if you do consider them a voucher then clearly define that for customers instead of being ************! Writing or calling in is useless because you won't get past the agent who's following a script and can't actually help you. Customer service is in the ground.