Printing apparatus that prompts a user to attach an attachment in accordance with the type or orientation of an envelope to be used in printing, and related method of controlling the printing apparatus and storage medium

An image forming apparatus comprising:

• a stacking unit capable of stacking a plurality of envelopes;
• an image forming unit that forms an image on an envelope conveyed from the stacking unit;
• a rear-end regulating plate, provided on the stacking unit, that regulates an upstream end of the envelope stacked on the stacking unit in a conveyance direction of the envelope;
• side regulating plates, provided on the stacking unit, that regulate both side ends of the envelope stacked on the stacking unit, in a direction perpendicular to the conveyance direction of the envelope and a thickness direction of the envelope;
• an attachment, removably mounted on the stacking unit, that is configured to be installed within the stacking unit, and that regulates an upstream end in the conveyance direction of an envelope stacked on the stacking unit by abutting against a trailing end of the envelope;
• an operation unit having a display, that accepts a size of an envelope stacked on the stacking unit via an instruction by a user;
• and
• a controller that controls the display, based on envelope information relating to the size of the envelope accepted by the operation unit,
• wherein the controller controls the display to display information for prompting the user to mount the attachment on the stacking unit in a case that the size of the envelope is smaller than a predetermined size.