From what I have read in posts by other sellers, it sounds as if you may have to choose a slightly longer handling time so that the ebay estimate gives you a couple of days more. that should cover you for most deliveries.
It is hard as sometimes the estimates are very optimistic but on the whole, I have found them to be in the ball park for most of my deliveries & I think it is because the sellers choose their handling times carefully.
It's a hard call for you in some ways as I think buyers do expect parcels to arrive within the estimated time frame so you do need to be mindful of that, but at the same time, you don't want to push the handling time or delivery dates out too far as that might influence a sale too.
