There are two postage bands and you can buy as many bits within them and pay one postage. These are the insurance levels sent by royal mail.
Domestic postage 
Goods totalling £0- £36. postage = £2.50.        £36- £500. postage = £4.50
International
Goods totalling  £0- £36. postage = £2.50         £36.-£500. postage = £6.50.