Harriet is very frugal, so she regularly combs the grocery ads looking for bargains. Usually she'll go to a particular store just to buy an advertised item, but sometimes she purchases other items that were not advertised as well. The purchase of the unadvertised items represent which type of sales?

Spillover sales.

Explanation:

Spillover sales can be defined as the sales of the product which are not advertised with the products that were advertised.

In this sales are done to the customers of the products that were not advertised while they have come to buy an advertised product.

In the given case, Harriet who always buys advertised grocery goes to a shop to buy them. But along with the advertised products she also bought those products that were not advertised. This type of purchase is known as "Spillocver sales".

So, the correct answer is "spillover sale".