To become a Virtual Assistant would also mean: to have an online payment gateway where clients can send their payments for your rendered services.
Me, as an Upwork freelancer, I am currently using the Upwork payment system. Clients are automatically billed each week, informing them of the hour usage, work screenshots, as well as the total amount for the rendered service(s).
I normally accept hourly-based contracts. I prefer working on an hourly basis, which means I am using Upwork’s time tracking system. Fixed-price contracts on the other hand are for repeat clients or on a case-to-case basis – which also goes thru the same Upwork system.