Usage Help On A Cover: Prexies + Certified Mail

I like this cover because of the Liberty-era certified mail stamp and the 1¢ prexie coils. I'm stuck on the usage though.

1955 Domestic usage.

The 15¢ certified mail stamp obviously covered the 15¢ certified mail service. That leaves 7¢.

The 1st class rate was 3¢. That leaves 4¢ to explain:

An extra ounce would only cost 3¢ which would leave a 1¢ overpayment - I do not think likely, but I do not have a better explanation.

Return receipt cost 7¢, so that probably isn't it.

Airmail was 6¢ so help there - also no indication that this was sent by airmail.

There are no markings on the back.

What does anyone think?

First impressions ... Are there stamp(s) missing?
Note the way the certified stamp and the leftmost coil are cancelled with a mute device. Then note the similar small black mark on the top of the certified stub in addition to the blank spacing above it.

That "black mark" on the certified mail label was a piece of fuzz that fell off as I took a closer look. I do not see any evidence of stamps that have come off, but that thought did cross my mind. If stamps have come off this cover, they came off very cleanly from a spot where a cancellation did not tie them to the cover while the ones remaining stayed securely affixed.