I don't see why you wouldn't just cut out the middle man charging you 2.5% and just go straight to the bank the check is drawn on to straight cash it?
The middle man is charging you $300 to call the bank the check is drawn on to confirm availability of funds, then will hand you the cash minus is $300. Where you could just go to said bank, they will immediately confirm funds on the account held with them and give you the full $12k cash.
I have NEVER had any bank refuse promptly to convert substantial 3rd-party checks to cashier's checks, and I have a) presented any number of four- and five-figure checks or b) charge me even close to 2-1/2% for the transaction.
You'll need to go to an officer of the bank to get it done. They'll call to verify the funds and can/will put a hold on the funds in the bank on which the check is drawn. They'll likely also contact the maker of the check to see if they actually wrote it, (This is where a delay, if any, was encountered, and that only for about an hour or two while the contact was made.)
Of course, you are going to have to carry with you a fair amount of personal documentation, but, if you are a law-abiding, semi-conscious individual, that should not be much of a problem.
If the check is that large, surely you can spend a bit of time in making the conversion...