Yes, Flaticon's resources are free and you can use them in t-shirts, invitations, post cards,cups, vinyls... Whatever you want!
If you have Free Plan, the only thing you must do is insert correctly the attribution in thefinal product. For example, if you use Flaticon's resources for a t-shirt, you must add "designed by Freepik" in the t-shirt.
Please remember that the copyright of Freepik's resources remains to it's author (Freepik), so you will not be able to register any image that includes any of our images.
"The User may use the Flaticon Content provided that it: (i) does not involve collective use, (ii) the Flaticon Content is not used in a manner that suggests an association or endorsement by Flaticon, (iii) the Flaticon Content is not used or included in a database or archive in such a way that could prevent or limit future visits or downloads from the Website (iv) does not resell, assign, transfer or sublicense the Flaticon Content or any derived work from a Flaticon Content, (v) does not use the Flaticon Content in printed or electronic items aimed to be resold, in which the Flaticon Content is the main elemant (e.g. t-shirts, cups, postcards, mouse pads, calendars, web models or electronic devices), (vi) does not use the Flaticon Content (totally or partially) in any trademark, logo or part of the same, of which may be used by any other mean to guarantee or to imply a guaranty of any product and/or service; and (vii) does not use the Flaticon Content in connection with porn, defamatory or any other illicit or immoral contents."