KanjiJLPT N2
Forehead

forehead, tablet, plaque, framed picture, sum, amount, volume

Mnemonic

A guest bows respectfully at the temple entrance but smacks his forehead straight into the wooden page — a framed plaque listing donation amounts hung just above the doorway. Now the guest has a throbbing red mark on his forehead, and his clumsy moment is immortalized on the page for all to see!
Additional thoughts
Visualize the guest bowing too deeply, cracking his forehead against the hanging framed tablet (page). Guest + Page = Forehead. The plaque also ties into the 'amount' meaning of the kanji.
Quick recall
A guest bows and cracks his forehead on the framed page plaque above the door.

Details

The keyword for 額 is forehead. This kanji refers to the forehead, the area of the face above the eyebrows and below the hairline. By extension, it also carries the meaning of a framed picture or tablet, as these are often hung at forehead height, such as the plaques displayed above the entrances of temples and shops. Additionally, it can denote an amount or sum, as in a monetary figure.
On'yomi
がく
Kun'yomi
ひたい