Instant concrete

Greetings, can anyone shed some light on what I should be looking for in an instant concrete please? It's just to set a letterbox post in. I believe some are not as good as others (prone to crumbling?).
There are about three different ones at my local M10 Mega; while the one in a ready-to-pour bag, where you add the water into the bag then pour the mix into hole, would be nice and tidy for me (without any other facility for mixing) perhaps it's not the best thing for the job.

Strength: I note there's one on here that seems cheap as chips and it turns out to be 20 mpa, whereas I know others are 30. I'm not au fait with strength and wonder will an extra 10mpa make a difference or are there other more important factors?

Setting time: I see they vary from 15 to 30 to 60 minutes setting time. 15 minutes standing on the berm holding a post and level seems the most attractive, could it be that that product doesn't have the best longevity and I would be better off setting up with a chair and thermos of tea/sammies for an hour's sit-in.

Volume: Depending on the brand it seems that I may need more than one bag but that seems like a lot; once you add water does the volume decrease a lot?

Your thoughts?

Any of them will do. 20MPA is plenty for a letterbox or use 30MPA if that's all you can get in fast set.

Just make the hole about 150mm larger total than the post size, if it is timber (75-100mm each side). Or about 200mm overall if it is a small metal post.

Have the hole nice and clean. Put the correct amount of water in the hole, or a little more. Put the post in. You might want to put a piece of brick or paver under the bottom so it's not sitting on the soil. Then add the concrete mix from the bag. Make sure post is level. You might need someone else to hold it as you pour as the bags are heavy. You can stir with a stick as you go if you like, but it's not really necessary.

Even the slow set ones will be firm after 15min or so. Enough so you don't have to hold it anyway.

I'm sure there will be a video online that will show you how to do it.

Volume doesn't decrease but there isn't a huge amount in each bag so don't overdo the hole size.

For a letterbox post, I'd be wanting to try some Sika Post Fix.
I don't know how much it costs - probably more than quickset concrete - but I'd give it a go.
https://www.youtube.com/watch?v=BLx4e_Y0eU8

The post needs to be plumb, not level.

I always put the mix in first then the exact amount of water as per the instructions, followed by a quick "rod" around to mix / use a spirit level to plumb and it will initially set in minutes so will hold to whatever you set it to.

I think your over analyzing this!

Thanks folks,

johotech: that was helpful, great to have some hole dimensions and useful re the paver underneath (I managed to acquire one of those).

mm12345: Interesting product!

budgel1: Ahh yes I see what you're getting at. I had intended it to be straight in both the vertical and horizontal but did not appreciate that there was a difference in terminology.

timbo69: I may well be; probably just going on experience from the current letterbox, the post for which perhaps wasn't set up the best and I felt that the concrete was not that great.
(being of the fairer sex this is not my usual gig). Promise I won't come here for advice on building a house ;-)