I'm going to let the post office know about my change of address.

Literal

Post-office [to-に] address-of change [object-を] notify-[intention-つもりだ].

つもりだ ('intention to X, plan to X') is the subjective-intention form — the speaker's declared plan. Distinct from ~ようとしている ('trying to') or ~予定だ ('scheduled to'). 通知する ('to notify') is a suru-verb for formal notification; takes an に-marked recipient. Bureaucratic register.