You appear to be a victim of a common scam: confuse the cashier and demand extra money. The usual advice to lessen the risk of falling for the scam is to do only one transaction at a time. Given the fast pace of selling drive-through foods, slowing down sometimes is difficult.
Whether your employer is acting legally cannot be determined by the facts in your post. You can review the information provided by Labor & Industries to see if your employer is acting legally. The L&I website is http://www.lni.wa.gov/WorkplaceRights/Wages/PayReq/PayDeduct/default.asp .
Note that the deduction for till shortage is allowed only in the FINAL paycheck.
If you believe your employer is not acting legally, you likely can file a complaint with L&I. Filing a complaint with L&I is free.
I notice that many businesses have surveillance cameras. I also notice that some drive-through stations have cameras. Does your business have cameras that recorded your transactions? Can that information be provided to law enforcement?