The 900 baht fee also includes your visa for entry to Myanmar and the minivan ride. It is possible to arrange to take your vehicle into Myanmar, at the border point if you have the correct paperwork.
I was there a few weeks ago. It is possible to take your own car. Also there is a parking area and a visa and transport service at the border which is very convenient and not too expensive. They never asked to see funds or onward ticket.
I did the border run. To Ban phu nam ron. It was so easy. I'm back home in Bangkok already. Thanks for the tips. Didn't ask for flight ticket or funds.
At immigration. Where you come into Thailand. You may have seen on this FB group that it has happened recently. On other groups it is happening fairly regularly now.
If you don't have the cash when they ask to see it they WILL refuse entry and send you back to where you came from. AT YOUR COST. The rules seem to be quite clear. Or you could take a chance.