I bought a dress in an online store, the parcel was lost at the post office for a month

Yes, the seller's refusal is legal. After all, in accordance with Article 9 of the Law "On the Protection of Consumer Rights", the consumer has the right to exchange goods of appropriate quality within fourteen days, not counting the day of purchase, if a longer period is not announced by the seller.

But you can go to court with a statement of claim. After all, in accordance with Article 32 of the Law on Postal Communications, postal operators are liable to users for non-performance or improper performance of services for forwarding internal postal items, instructions of users regarding postal transfers. For untimely delivery of registered postal items of all types - a fine in the amount of 25 percent of the cost of the postal service.

Please note that statements and claims regarding such cases are submitted to the postal operator within six months from the date of submission of the postal item or postal order for forwarding.