Originally Posted by mozilla

While in the UK, try using a VPN with an endpoint in the US, preferably a corporate VPN if you have one, as MP SHOPPING may block the popular consumer VPN services. This will especially be useful for the shops you encounter after clicking through from MP SHOPPING, as many won't be so kind as to ask where you live, they will block you outright instead.

Note that shipping out of the US is often an exclusion ground for MP SHOPPING benefits. Additionally, using an IP that doesn't match the ship-to country is a good way to get caught in non-sophisticated "fraud" filters, especially if the IP belongs to a consumer VPN service. So ship to your US address whenever possible.