But it is said she has been kept busy over the summer, working and saving money before her move to Fife, Scotland.
Louise – whose parents are Prince Edward and Sophie, Countess of Wessex – has been spotted greeting customers and helping out on the tills.
She has reportedly been working at the centre for several days a week – and has been doing so ever since she finished her exams in June.
One shopper who says the royal served them on two occasions said they "had to look twice" when seeing the Queen's granddaughter.
They told The Sun: "She is a really modest and sweet young woman who is polite and attentive to customers. She seemed to be loving the job.
“You’d never imagine the Queen’s granddaughter would take on a role working behind a till.”
Another added: “The staff seemed to adore her.
“It’s not every day you buy your begonias off a royal.”
Ingrid Seward, editor-in-chief of Majesty Magazine, said: “Isn’t it wonderful the Queen’s granddaughter rolled her sleeves up and got her hands dirty with a summer job before going to university — just like any other normal teenager.”
The 18-year-old will start her degree course in September.
She follows in the footsteps of Prince William and Kate, Duchess of Cambridge who first met at the Fife university.
While Louise will study English, Kate read History of Art and William studied Geography.
Buckingham Palace has not revealed Louise’s results, but St Andrews is one of the country’s top universities.
Standard entry grades are usually set at AAA and minimum entry grades of ABB for the English MA.
A Palace spokesman said: “Having received her A-level results today, Lady Louise will start at St Andrews University in September to study English.”
Lady Louise Windsor will attend St Andrew's University later this year Andrew Matthews
Lady Louise Windsor with parents Prince Edward and Sophie, Countess of Wessex Johnny Green