Of course you are, and they can detain you until the police arrives.
So, the store wants to look in the store bag before you leave the store. Who cares? Takes 5 seconds.
I worry about lots of things, but some guy checking the bag to see that I didn't steal anything in a store is no big deal to me.
Your opinion may differ. I'll watch as you assert your rights, but just briefly before I go to my car with my purchases.
They can try. They'd also better hope to be really convincing in their argument that simply ignoring the request for a search, absent any other evidence of wrongdoing, is itself reasonable suspicion, or they can deal with the consequences of unlawful detention.
Look, I'm not advocating anyone try this, merely pointing out that merchants do not have carte blanche. If the principle is worth the hassle, go for it. If your time is more valuable to you, give up the receipt. Pretty simple ;)