Franking is any sort of application of non-stamp postage, not just doing it for free.
When Frankie, Nina, Jane and Inspector Prescott are examining the postal bar code, they determine that it is a "forgery" based on the low quality of the printing (they speculate that it was printed in a dot matrix printer), implying that only the Postal Service can print these codes. That is not true. Anyone can print a postal bar code; WordPerfect and Microsoft Word both give users the capability to print them on envelopes and mailing labels.
During the Karaoke Session at the end the "wireless Mics" were corded mics without cables you can see the connector pins when they are singing.
Maura would know better than to say "how fun".