What type of mailing modification could cause delays in acceptance of mail at the post office?

Applying incorrect postage is a significant factor that can lead to delays in the acceptance of mail at the post office. When the postage affixed to a piece of mail does not meet the required amount, whether it's too little or incorrectly calculated due to weight, size, or destination, postal workers will not accept it for processing until the issue is rectified. This means that the sender must either add more postage or correct the mailing criteria to ensure compliance with postal regulations.

In contrast, identical pieces of mail, standard packaging, and the inclusion of standard return envelopes do not inherently cause delays. These factors align with postal guidelines and can even facilitate faster processing as they help streamline sorting and handling by postal staff.