Fonts: You can use fonts for Print On Demand (POD) use - as long as the text is flattened, and un-editable so that the font file isn't needed anymore.
Designs: Neither our Premium License nor our Corporate License allows designs to be used as-is on any POD website.
To use purchased graphics or designs in either a digital item, or in an item you'll be sending to a third party to create the final physical product (printers, manufacturers, print-on-demand sites, etc.) you need to edit the purchased graphics and combine them with other elements (graphics/text) so that the purchased items become just a part of a new and unique transformative design.
The new design cannot include the original graphics in their as-purchased state, and the new design can't be similar enough to the original that it could be confused for the original, or compete with the original in the marketplace.
Since POD sites are intended for original artists and designers to display and sell their own work, these requirements do mean you'll need to put in some design effort and skill to create your new design. It also means that some designs and illustrations may be so distinctive and unique already, it's impossible to modify them enough to create something truly new and transformative from them.