The OP is not being treated unfairly. This is an online purchase, as with everyone else OP has a set number of days in which to inform and return the goods, the minumum requirement by law is 14 days to inform of return and a further 14 days from that to make the return, most of us operate on a 28 or 30 return policy to encompass both notification and return to the maximum limit required
The company OP used has given the OP a voucher/credit note even though they could actually have charged OP for not returning the goods within the time or refused to take the return full stop. Had the goods been purchased in a bricks and mortar shop, they don't have to refund at all for a change of mind, they can refuse to accept any return except a faulty good. Distance selling is different, people obviously need to see the item to make a final decision, 14 days after receipt of the item to make a decision is fair, a further two weeks to package and send the item is also fair.
This business went above what they are required to do in offering a credit/voucher.
How is this unfair?
Is a month not enough, what would be long enough 3 months, 6 months, 12 months, at any point someone wants to? There has to be a cut off point somewhere otherwise no business could function in any kind of distance selling.
Many many products particularly fashion and tech have a very short shelf life. The costs of returns are high in fashion and accessories, that has to be passed on to future customers, this isn't one small return, it is one of millions on a daily basis across an industry, it affects us all because those costs have to be factored into new product pricing.
This isn't all on the shoulders of the OP, this is just a small drop in the ocean but in terms of being treated unfairly, no, this isn't unfair.