A few times I've told a client a price and when they come for pick up they hand me cash. I don't count it until after they leave and realize they are short either $10 or $20. Would you just let it slide or do you call them know and ask for more money? I'm not one for confrontation but my husband thinks I should say something.

I cannot address the transactions which have already been completed, but be sure and count your money as soon as you are paid, even if you must go to the lady's room to do so!  If you find that you have been shorted, immediately ask for the balance of the payment.  

Whenever you check out anywhere, they always count the money. Not only for your protection but for theirs. What if they paid you too much? Just count it in front of them and don't be embarrassed to do so. It is a common practice. 

I would probably let it slide but make sure it doesn't happen again.

100% count the money before they leave with their cake.  If you went somewhere to purchase something with cash, they certainly would make sure you paid the appropriate amount before they allowed you to leave with your items, so why should this be any different?  Do not let people take advantage of you.  By not double checking the money given you are only hurting yourself and your bottom line.

As for the transactions already made, there isn't much you can do.  But for any future clients, count that dough!

It's too late to change what has happened. But learn from your experience. Going forward, for your protection, please follow common practice:

You're running a business, and with all businesses, money is paid for a product or service. If it's a cash transaction, that money is counted in front of the customer.

I sell very few cakes, but I was referred by a friend to one of her customers whom she was unable to accommodate.  Both cakes I did for this lady were seriously under priced, but I did realize it at the time.  The first cake I priced at $85.00 and she paid me in cash - $75.00.  I immediately let her know she had shorted me by $10.00.  The second cake was priced at $65.00 and she handed me d$60.00.  Again, I reminded her of the agreed upon price.  I do not believe it was accidental.  She is no longer a customer.