quiltnmom, I would not be concerned about whether they took personal offence. They are in a business and should be fair and straight with their customers. The fact that they didn't tell you about the sale coming up in 1 week, I myself would take as a personal affront.
First I would show my receipt and ask for the sale price since I only bought it a week ago. If they agreed, I would return the blades, explaining that you were distracted and didn't realize the price till you got home. If they refused, I would return the machine and the blades.
Then I would come back into the store and buy the machine on sale.
Hugs and smiles, Linda aka wildyard