Naming and folders
Use lowercase, hyphenated filenames and a flat folder structure no more than two levels deep. Buyers unzip on every OS, including ones sensitive to spaces and special characters.
Preview images
Upload a cover plus extra images from the product form: one hero shot, one showing the product in context, and one showing what is included. Covers are shown in search and on cards, so make the first one legible at thumbnail size.
Compression
Zip the whole package rather than uploading loose files. Use standard zip, not proprietary archive formats — some buyers are on tools that only unzip .zip.
Size limits
One product file of up to 50 MB, and images up to 5 MB each. Bigger collections sell better as several products in a bundle anyway, because each part gets its own listing in search.