The rule for FRE's is -- It has to close within its own folds, you're not allowed to make it into a box by cutting or modifying it in any way. The adhesive strip has to be adhered to its mating surface. You're not allowed to add any material to make it larger.
You should always add additional tape to all the seams, the FRE's are just not that robust.
Postal clerks are allowed to refuse over-stuffed FRE's, at their own discretion, if they feel that they can't be shipped safely without danger of bursting, so use additional tape.
A couple of other comments -- the new legal-sized FRE's hold more and ship for the same price -- $4.95 if purchased at the post office, $4.75 if postage is purchased online.
The padded FRE's are the same price for postage whether purchased online or at the PO - $4.95. For certain items these can be handier, but these are meant for businesses, so the post office normally doesn't have them. You can order them for free from usps.com.