Pop up Shop Poster budget and pricing

So after designing the posters my attentions turned towards actually pricing the posters accordingly to take into account the cost of materials, printing, the time I had put in and the commission that was going to be taken away.

Materials
Paper (50p per sheet) x 15 = £7.50
Printing (£1 per sheet) x 15 = £15
Ribbon or securing band (10p each) x 15 = £1.50
total = £24
Price per poster = £1.60

Time
£6 per hour
Number of hours: 10 - This is based on the amount of time I spent designing and how long it takes to print a batch of 15.
Total £60
10 hours divide 15 posters is 40 minutes.
1 poster is £4 in time.

In order to be in the pop up shop I had to pay a one off charge of £10 for security fees and I will also have to pay 20% on each poster I sell.

So the £10 charge is divided by 15 to get 67p which is added on to the price of each poster.

So to recap one poster would cost;
materials - £1.60
time - £4
security charge - 67p
so the total would be £6.27p

I must also add the commission on to the price of the poster to make sure I don't leave myself sort.

The 20% commission works at £1.24 on the current price so the RRP is £7.51 which I will round up to £8 to make my selling price.

So each poster will be sold for £8 each and I will be selling the set of three at a discounted price of £21. I know I can break even quicker selling the full set of three so I decided to take the hit on the full set.