Game dev refuses to correct mistake through purchase

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PLEASE NOTE I AM NOT SURE WHAT ‘ TAGS AND FLAIRS ‘ARE APPLICABLE IN THIS CONTEXT

Location: south Africa
I am dealing with a game dev from Kazakhstan. I had made a purchase while on beta and did not receive the item/ perk I’d purchased. Once the update was released to public the price went from 11usd to 17usd then to 16usd , when i had asked , shown them bank statements proving POP ( proof of purchase ) they dismissed me saying that because I was not invoiced and that they can not assist , I had made purchases on other games before and had invoices , tho this purchase did not present one , a problem on their end ? Perhaps… I pleaded with them to do recon and check their bank but they hadn’t, and are insisting they are unable to help.
I ask for some guidance on the matter as I had spent R200 ( about 11 or 12 usd at the time ) and have nothing to show for it but a transaction on my STAMPED ( approved ) bank statement. Is there anyone who’d be willing to assist

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