Yes, Freepik'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 on the end product. For example, if you use a Freepik's resource for a t-shirt, you must add "designed by Freepik" on 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 Freepik Content provided that it: (i) does not involve collective use, (ii) the Freepik Content is not used in a manner that suggests an association or endorsement by Freepik, (iii) the Freepik 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 Freepik Content or any derived work from a Freepik Content, (v) does not use the Freepik Content in printed or electronic items aimed to be resold, in which the Freepik Content is the main element (e.g. t-shirts, cups, postcards, mouse pads, calendars, web models or electronic devices), (vi) does not use the Freepik 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 Freepik Content in connection with porn, defamatory or any other illicit or immoral contents."