Interesting about adding VAT to shipping costs. I've just gone back over my receipts from various auctions, and it would appear that whilst Spink & DNW add the VAT to carriage, at 20%, London Coins don't. They have consistently charged a fixed amount of £7.50, for several years, including on my latest invoice dated 9.3.17. The cost of next day tracked delivery is £6.45, therefore £6.45 + 20% = £7.74. As £7.50 is charged, that would clearly indicate that VAT is not being added to postage, and that the additional £1.05 is for the jiffy bag. Colin Cooke didn't add VAT to postage either.
As far as the £5 a day storage costs are concerned, I'd say that if, as John did, you've settled up as soon as reasonably practicable, eg: the day after the auction ended, then charging storage costs is bloody outrageous. If it's been a few weeks without settlement, then maybe at that point they could kick in.