It is normal for them to charge you a 2 or 3 percent commission when cashing a ticket. Alternatively you can take the ticket to the lottery office in Bangkok and cash it with no fee.
Also I have heard that GSB, Om Sin, will cash tickets for you if you have an account with them.
Also be aware that some lottery tickets now have a charitable donation(charge) on them. So if you win that amount is subtracted from the winnings too.