Won't charge you if the leak ain't there, that's a load of bs, haven't you already paid them to stop the leak, if so, they need to repair the leak, they can't double charge you, possible that the rear main seal has started to leak but it would clearly be visible, it's not like there's an airtight seal between motor and box, with the amount of oil you say it's leaking, one quick look under the car and you'd be able to see the oil leaking between the motor and box.
If it's in fact the rear main seal, those are cheap anyway, not like they're gonna have to spend alot on it, a good quality seal should cost between R150-250 I use the febi seals on my personal vehicle's, they've never let me down.
Basically, box comes off + (if manual) clutch, pressure plate and flywheel, then you look directly into the seal, I usually take a sharp object and hit a hole into the old seal, then use a tool with a hook, put into the hole and pull the old seal, gently tap in the new seal with a mallet.
The above is only if the seal does come loose from the housing, some engines has them bonded to the housing and only sells as a unit like the 2003-05 VW polo's for those you have to loosen the sump to fit.
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