Agree that 300mb is way too small 😞
However I have found a better workaround than a readme file.
I host my digital product files in AWS S3 so that the directories/buckets cannot be listed. But the files have read permission so they can be downloded.
Then for my digital product file I use a .url file with the following contents: [InternetShortcut] URL=URL/Address/to/S3/file.zip
When the customer clicks the .url file it will download the zip containing the real product from s3.