You can use any font you like, provided it is suitable for HTML. Please test it well with the preview function in the top right corner of the editor.
You can upload your fonts in the portal or in the editor.
If you want to use bold or italic fonts, be sure the uploaded fonts support this functionality.
Pay attention to readability: small fonts are not good for your conversion. Choose a font that is easy to read. If necessary, shorten the text to make the letters larger.
As with images, the hosting should be able to handle a lot of requests. A Google Drive folder or something similar is not enough.
Google Fonts is a good base, with more than 1400 fonts and reliable speed and availability.
You can also download the required fonts and upload them to your account.
Similarly to images, any order with a font that does not return a 2xx HTTP status code will fail. This is to ensure the quality of your orders and postcards.