One common 'shoplifting' scam involves having the cashier ring up an expensive product as in inexpensive one. This is often done by a "shopper" and cashier in cahoots, though it may also be done by a shopper who affixes a phony UPC label on top of the legitimate one.
To avoid allowing such crooks
to escape undetected, I would suggest using several different types of tags, which could be chosen based upon product price, but could also be varied somewhat based upon product classification. When a product is rung up, the register would send out a "zap" suitable to deactivate the type of security tag that product should contain. If the product contained a tag of another type, that other type of tag would set off the alarm when the "shopper" tried to exit the store.
There are many ways this type of system could be implemented. The most sophisticated would involve using RF transponder chips. These would allow the greatest precision and spoof resistance, but would also be the most expensive method (both in terms of equipment and in terms of per-tag cost). An alternative would be to simply use tags tuned to a few different frequencies and have an exit gate scan for all of them.
Perhaps the best variation on this concept would be to have the less expensive product classifications use one frequency tag each and the more expensive ones use two. If there were five frequencies F1 through F5, they could e.g. be allocated:
F1 -- under $10
F2 -- $10-$25
F3 -- $25-$50
F4 -- $50-$100
F5 -- $100-$250
F1+F5 -- $250-$500
F1+F4 -- $500-$1000
F2+F4 -- $1000-$2000
F2+F5 -- $2000-$5000
F3+F5 -- $5000 and up
With the frequencies as indicated above, it's possible to allow the system to issue the "zap"s for two consecutive price levels without zapping a more expensive product.
Using this type of approach, it should be possible to substantially reduce shrinkage with only a slight increase in equipment cost and a minimal increase in product cost (products over $250 would require an extra LC security circuit, but since it would probably be possible to incorporate two LC's into a single tag that should not be a problem).