From the announcement:
Royal Mail rules on sending water-based paints have changed and no more than four containers of up to 150ml each can be sent in one parcel, whether in the UK or internationally.
That means we must break down orders for paints into groups of four and send each group separately in order to comply with the law. The shipping charges have been changed to allow you to select Royal Mail shipping for 4 pots or less. This applies in the Uk or internationally.
In the UK for more than 4 pots please select 5-8 pots UK (which will be sent in two parcels) or UK Courier.
For an order of more than 4 paint pots outside the UK please contact us for a quote, or we will contact you with the additional postal cost.
We would encourage you to order from a retailer in your country or region as that will probably be much cheaper.
This is a complete pain, but we have to comply. I am looking at alternaives and a group of traders are lobbying Royal Mail