Makeup Brush (used with foaming cleanser)
You can use this brush with the foaming lash to cleanser the whole eye area prior to lashing. They're nice and soft, but not so floppy you can't get anything clean. Wooden stick with synthetic fibres. Keep the possibility of infection as low as possible by cleaning the entire eyelid and lashes.
Try selling it to your clients with the foaming primer/cleanser so they can take care of their lashes in between refills. Clients who clean their lashes regularly will have better retention.