With two decades of professional expertise, I often provide free advice to not-for-profit organisations and small businesses.
Having been vegan for a similar amount of time, I have also created and managed a number of vegan projects and events, given talks and written articles - all of which have been free to access. And I have volunteered for many more vegan initiatives.
While free to the respective beneficiaries, these all have a cost to provide (whether in time only, or financially as well). And while such costs are often small, they quickly add up.
So, if you would like to support initiatives such as free professional advice and free vegan events (particularly if you have benefited directly) you might like to consider leaving a "tip".