A Belgian company should be able to accept payment via IBAN/BIC. That's a free money transfer service to anyone living in the Euro zone or holding a bank account in a Euro zone country.
Trouble is, most UK banks charge upwards of £20 for the service plus currency exchange commission. Most British people don't know what IBAN/BIC is, so perhaps Carsoft wanted to offer a better-known alternative?
However, PayPal is the best known alternative to IBAN/BIC. It is widely used by eBay sellers located in the Euro zone who offer goods on eBay UK.
I would be very hesitant about using Western Union as it is much less secure than IBAN/BIC and PayPal, and getting your money back in the event of a dispute would not be easy.