If you notice this happening it may be that an additional charge was setup for the product which is getting added to the price when the product is added to your cart. To check this:
  1. Go to Fundraising > eCommerce
  2. Next to the product in question click "Edit"
  3. Go to step 7 "Product Options", this is where various options about the product can be setup. If you don't see any product options here contact support to further investigate the issue.
  4. Next to each product option click "Edit"
  5. Examine step 4 "Choices for this Option". If there is any value under the "Additional Charge" column, that means that particular choice for the product includes an additional charge that is added to the cost of the item when it is purchased. If this was not your intended purpose, remove that value.
This functionality is setup to allow for you to have different pricing for a single product based on particular configuration options. For example, if the product is a t-shirt you might have an additional charge if the constituent selects a larger size or a particular color.