This is a common question, as you are not the only person who sometimes wears disposable contacts for longer than recommended by the manufacturer. In general, it is always best to use medical equipment as prescribed or recommended, simply because it's not worth risking a possible adverse event that could influence your health from a product, medication, or device that is no longer functioning properly because it is too old. In the case of contacts, it is always possible to develop an eye infection
from contacts that are not cleaned or replaced properly. In your situation, the best thing to do would be to discuss your concern with your eye doctor
. He or she can go over the exact brand of contacts you are using and give you a general sense of whether or not it may be okay to use them for slightly longer than recommended on the box. You are right that the manufacturer certainly has an incentive to want you to buy more contacts, but before you decide to go against that product label, you should talk things over with your eye doctor (who is not selling you the contacts) to get a sense of any risks that may be involved.