Do not use Mears in any form. The only way I would ever deal with them is because they operate Disney's Magical Express. And that's only because you have Disney as the middle man. Mears is notoriusly unreliable. Case in point: they did not show up for our return trip. However this was via DME, so when they didnt show Disney grabbed us a cab, prepaid for it, and sent us on our way, no muss no fuss.
You can find dozens of horror stories about Mears.
To get around WDW by taxi, we average about $20 with tip. We do that when we have an early breakfast at a park way before opening or to get to another resort quickly, usually to dine.