No sense paying for 12 oz, if the package is only 6 oz. Also, I'm not sure the rule on delivering. maybe the PO would refuse and return a package with insufficient psotage or I've heard of them trying to collect it from the customer. Either way, it can simply be avoided by entering correct weight and paying appropriate postage.
USPS offers frre front door pickup, which is scheduled online. However, being in a gated community, I don't know your rules.