That is a really good question. So must third-party payment gateways do require you to be a registered business. This is for legal reasons as they need to make sure people who are using their credit card processing capabilities are legitimate businesses and not committing fraud using their tools.
Alternative payment options such as Cash on Delivery do not require you to have a registered business as there is no system in place processing that transaction - you are physcially collecting cash from your customer when you deliver the product.
I would recommend reaching out to a local lawyer or business expert to see what the legalities for running a business are in Bosnia and Herzegovina, especially in relation to the collecting of tax.