The fifth dimension... found!

It so happens that a large box of Cheerios can actually be a gateway to the land of the lost (peppercorns). However, it seems a different portal is required to access the dimension in which letter-openers are sequestered.

(I know, I know, why on earth was a toddler allowed to handle a letter-opener? Choose two of three:
a) He's quick.
b) I'm a bad parent.
c) 'bout time the boy started earning his keep 'round here. Least he could do is sort the mail.)