A television ad for savory sandwich instructs its viewers to look in their mailbox for coupon offers that will save them money on their next sandwich order. this is an example of

advertising support

Explanation:

This is an example of advertising support where the coupon offers sent to potential customers mailboxes support the main marketing effort. Another example of advertising support are product displays placed on retail stores.

It is an example of old school marketing, now this is done through spam emails. There is no way that a company knows who is watching their commercials so they will simply have someone place coupons on people's mailboxes.