It refers to that kind of promotion where you buy a product and get another free product attached to it, either when you buy a given size of, say, soap powder and there's a small bonus pack attached to it, or when you buy a toothbrush and there's a free tube of toothpaste attached to it, for example.

"Los "bandeos" son productos que se venden con una oferta adjunta. Por ejemplo, cuando se vende un paquete de espaguetis acompañado de una salsa de tomate."
http://www.elsalvador.com/mwedh/nota/nota_completa.asp?idcat...

In fact the "bandeo" seems to be the attached bonus product:

"Rico Café, ofrece bandeo."
http://www.univo.edu.sv:8081/tesis/021169/021169_Cap4.pdf (p. 87).

And that's what was happening in your text, I think; this worker was allegedly detaching and stealing the bonus products.

The technical term for these, in the US at least, is "banded products" or "banded packs", and "bandeo" must come from there, I imagine:

"Another type of price deal is the bonus pack or banded pack. When a bonus pack is offered, an extra amount of the product is free when a standard size of the product is bought at the regular price. This technique is routinely used in the marketing of cleaning products, food, and health and beauty aids to introduce a new or larger size. A bonus pack rewards present users but may have little appeal to users of competitive brands. A banded pack offer is when two or more units of a product are sold at a reduction of the regular single-unit price. Sometimes the products are physically banded together, such as in toothbrush and toothpaste offers."
http://definitions.uslegal.com/s/sales-promotion/

"Banded Product
Definition: When two or more items of the same product are bound together as a single package and offered at a reduced price to consumers."
http://www.moneyglossary.com/?w=Banded Product

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Note added at 43 mins (2013-07-03 17:05:17 GMT)
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So strictly speaking (in the US), if the "bandeo" is an extra amount of the same product, it's a bonus, and if it's another item (like tomato sauce with spaghetti or toothbrush with toothpaste), it's banded.