Steps for going shopping with your bike:
Get a backpack.
Get on your bike and ride.
Go to the store.
Lock up your bike.
Walk into the store with your backpack on.
Get a cart, or if you aren’t buying much then just carry the items you are buying.
Put items in the cart, not your backpack.
Make sure to not buy more than will fit in the backpack.
Go to checkout and pay for your stuff.
Put items in the backpack after you purchase them, using no paper or plastic bags.
Put backpack on and walk out.
Unlock bike.
Ride home.
Unpack your stuff.

This shopping run involves no pollution or greenhouse gas emissions from a car, no trash from disposable bags, no paper or petroleum used to make disposable bags, and some nice exercise for the shopper.  I have been doing this for the past few months at most stores within three miles, Including Target, WinCo, Costco, Home Depot, Taco Bell, Carl’s Jr, Starbucks and Orchard Supply.  Bringing a backpack into the store is not an issue at all, which was a surprise to me at first.  I expected people to tell me I couldn’t bring it in, or being told to empty it out at the register and prove I wasn’t stealing anything.  No store employee has ever said a word about it.  Also try this trip with two people, you could enjoy the ride together and carry twice as much stuff.