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So instead she makes her own.
Now she can drink her fill for a fraction of the cost.
Kombucha is a mildly fizzy fermented tea, and most of the materials needed to make it are simple to obtain.
A large jar, sugar and teabags can be gathered without too much trouble, but the star of the show needs a little more preparation.
The engine of the kombucha-making process is a symbiotic culture of bacteria and yeast known as a scoby.
Knowing somebody who already makes kombucha is easiest, as a scoby can be split up once it has grown large enough to start new cultures.
Mrs Marshall made hers from scratch by buying a commercial bottle of kombucha, adding tea and sugar and waiting a couple of weeks.
"Now that generates three litres of kombucha every week to a fortnight all for the price of a cup of sugar and four teabags."
The resulting brew can be flavoured by adding fruit or spices such as lemon or ginger to the fermenting mixture.