I think the only way to help users make less mistakes is better visual aids on the website, possibly bigger font for minimum amount. Brighter claimer to download and verify the letter of guarantee.
You could create an alert, when the person clicks on "Mix Coins", a popup will appear alerting you to the minimum sending.
One question: if the user sends all transactions of 0.0005, will it count or not?
A notification with a warning in a larger font or a popup if possible are good ideas.
I remember on other platforms there is a notification that if you send below the minimum amount it will be considered a donation.