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You would be fine traveling with groceries for thirty minutes as long as the temperatures weren't extreme, such as hundred degree weather. Many grocery stores have brown paper books or insulated cloth bags made specifically for cold items at the grocery store. You could consider using either one of them to keep your items cold during the bus ride if you are worried about food spoiling. However a thirty minutes bus ride isn't much different than a thirty minute car ride to the grocery store which is an acceptable distance in many rural towns where people must travel to the city in order to find a supermarket.

As long as it's not a heatwave, you should be fine with a 30 min travel time, you can also get insulated grocery bags and carts to help keep your food cool.

Here are two examples of insulated grocery bags

http://www.walmart.com/ip/Quest-Eco-Friendly-Insul...

http://www.reuseit.com/Market-Baskets/Market-Baske...