Wheelie,
I've had a couple of echecks in my time, but not lately. I do not post until it clears because Paypal is taking the money out of the buyer's bank account, so if there are no funds the echeck bounces and if you have posted, well TT. If the buyer has a credit card attached to their Paypal, an echeck doesn't happen, as Paypal makes the payment instant in this case because they know they can access the credit card in the event of not enough funds in the bank.
It has worried me that the buyer may not know that this Paypal procedure and time frame is so long winded and complicated, so I have messaged them and let them know that due to their funds being taken out of their bank account, the payment is delayed and when cleared, the item will be posted. I may mention Paypals clearing timeframe, but say, hopefully it will clear quicker than that. When it clears, I message them and let them know it has finally cleared and that I have posted it.