If it's a fiancé visa then you should not have a letter with it and there's no need to go to a post office because there's nothing to pick up.
The letter and the post office only applies to visas valid longer than 6 months, As your visa is only valid for 6 months, it's just a sticker in your passport.
To note, the above comment is the first time I've heard of someone needing to show the letter to immigration - usually it's only the passport that is needed, as far as I'm aware, unless they are doing things a bit differently now.
So:
Fiancé visa:
- 6-month visa sticker
- no letter
- no BRP
- no need to go to a post office
Spousal visa:
- visa sticker valid 30 days
- BRP ID card valid 33 months
- letter arrives with the visa
- you have 10 days from entering he UK to pick up your BRP card at a local post office, which you arranged when you applied online.
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