Carrying heavy shopping or other loads on a bicycle is not a particularly easy task. Store bought trailers can be rather expensive not to mention bulky so if you have the tools and some time perhaps creating your own bicycle trailer is the way to go.  You can use an old backpack frame and some BMX bike wheels as seen here or perhaps this bamboo trailer might be more appealing.

If building a bike trailer is not your thing check out the carry anywhere suitcase bike trailer from the same site. It weighs only 5kgs and can carry 45kg. It’s pricetag of $500+ however might just inspire you to take the DIY road.