At the moment, we're unable to remove any design elements (like flowers or borders) from our paper designs as they're built into the template by the artist and cannot be edited.
If you have a vision in mind, you can create your own design and upload it onto Zola!
For envelopes, if you want to remove the design, a workaround could be to select a color envelope that isn't white or cream, as it automatically removes those design elements.