How to correctly calculate postage?? help please

[dietstartstmoz]

This week I have sold 5 lots of toys in an effort to declutter. I calculated the postafe via the ebay link to the royal mail calculator. All items were sent in boxes, and I weighed them all at home and obtained the postage costs and used this when listing.
When I went to the Post office yesterday they charged me approx £2 extra per item-what have I done wrong.

I listed them all as a packet-is this wrong? I have other items listed and am now panicking that I have undercalculated the postage also-the idea is to declutter and make a few quid for xmas.
All advice gratefully received
Thanks

Tell us how much they weighed and what size the packet (or box) was.

OK, I have an item selling this weekend. It is in a small box, approx 20cm high, 34cm long and 24cm wide. It weighs approx 1700g.

Royalmail.com price calculator has it as £9.25 (signed for 1st class, less than £50 value).

Postal-rates.co.uk have it for £6.42 & 77p recorded delivery=£7.19

Help!

I've got that listed as £7.40, 1st class. Add 95p for Signed For = £8.35.

With packaging, you would be looking at around £9.

Wish I'd seen this last week. I've just seriously under calculated. Sold a playmobil thing for £6.70, plus £2 postage. Cost me £6.95 to post. The worst one was a huge box, I thought tenner for posting, cost £22.95. I think I go a bit mad with bubble wrap.
Hey ho. On the positive side, I have decluttered!
And I've another 5 or so sets to list, so I'll use this calculator! Thanks!

Linerunner-how did you calculate this?
I think i will go to the postoffice tmoz to check it. How can 2 royalmail websites give such different prices? Nightmare! The aim is to make a few quid not just give it away

I've got the Royal Mail Postage Rates Calculator page!

It's normally very good, unless my kitchen scales have been a bit wrong.

I use the postie app for iPhone to calculate postage accurately. I also weigh on my electronic kitchen scales with the packaging on top. Anything over 750g goes via courier either Collect + or Hermes.

I also measure carefully to make sure packets and letters are within size. Did once get a miserable cow jobsworth at the post office who refused to post as a letter because she "can't push it through the slot" - despite me getting it through just fine on the post office sizer thingy. I did have to gently push rather then just will it through though I paid online for my postage and put it in the box and hoped it wouldn't get lost in post but it was low value and worth the risk.

I make your parcel £6.50 first class standard and £7.45 recorded. Standard parcels is £5.30.

Cheaper to courier. Hermes will do it for £4.19 I think from memory and collect from you.