"Scríobh sí a sheoladh ar an litir agus chuir sí stampa sa chúinne."

Translation: She wrote his address on the letter and she put a stamp in the corner.

[morvan82]

She should have put the letter in an envelope first, and then put the address and stamp on that.

[SatharnPHL]

You're obviously not old enough to remember aerogrammes, where the letter was the envelope.

[FionaOnDuoL]

Also she forgot to write her own address on the back.

[SeumasO]

Why not a seoladh??

[SatharnPHL]

a seoladh - "her address"
a sheoladh - "his address"