I never give the solar oil away for free, as said previously they won't use it or have any respect for it if they are given it for free, and it will be a bit of a kick in the teeth to pay for a subsequent bottle if they were using it and ran out.
I only sell the large 15ml bottles in my salon and have never had any issues selling them.
I sold over 25 bottles of it this December and I am a one man band who doesn't solely do nails (although they are my main service)
It all starts with your consultation. Look at the clients nails and tell them what you see for eg "Mary I can see that your nails are looking a bit dry do you suffer from peeling nails/ do they break easily?" Find out what her long terms goals are "would you prefer to have instant length with enhancements, or would you like to promote your natural nails to grow with shellac/ manicures"
Now that you have found out what the client really wants you can be prescriptive in you recommendations for eg "well if we are going to grow your nails it will be very important that you use solar oil at home to keep them strong and flexible, it will also heal split skin / hang nails to make them look their best for longer" or if I were doing enhancements " it will be important that you use you solar oil to keep your enhancements flexible and strong and it will also help to keep your natural nails strong underneath them and the stronger your natural nails are the better your enhancements will last" etc
I have told my client all about the benefits of solar oil and got their interest in the product before the treatment has even started , I then chat away as normal for the treatment and apply oil at the end saying " this is the oil I told you about , this is how you use it , keep it up by a bedside table or somewhere you'll see it and apply twice daily"
I always word things with the assumption the client will buy it but I am not pushy in any way , I do not mention the price unless the ask and then I will say its 15 euro and will last you 6 months , my prices are clearly displayed anyway, 7/10 clients will say at the end if ther service they would like to take a bottle , 2/10 won't say anything so I give them a little nudge with would you like to take a bottle today ? And then of course there is always one who just won't budge
c'est la vie