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Well-l-l-l, that's a bit of a puzzle. Calendula are annual plants, meaning they grow, flower, and make seeds in one season (year.) Some plants are biennial, meaning they spend their first year growing, and the second year producing flowers and making seeds. Half annual, or semi annual, means 1/2 a year, or every 6 months; it can refer to interest compounding, or magazine publication , but I've never seen any reference to half annual being applied to plants of any kind. Now, some varieties of calendula are described as half double, so perhaps there's a misprint on the label. In case there are other errors, here's a piece on calendula: https://www.gardeningknowhow.com/ornamental/flowers/calendula/growing-calendula.htm

Calendulas are classified as "Annuals or short-lived perennials." It's winter survival is poor in colder regions nor does it tolerate extreme heat. However, this doesn't mean you can't try and see what happens - I would go ahead and cut it back and see if it comes back the following spring.

For more information on growing calendulas, please visit the following link:
https://www.gardeningknowhow.com/ornamental/flowers/calendula/growing-calendula.htm

Calendula officinalis will reach 1 to 3 feet in height and spread out about 10 inches to 2 feet wide. These measurements may depend on the specific variety and how closely they are spaced.

You should have no problem growing Calendula in your garden. Look for a planting location that is morning sun instead of the hot afternoon sunshine.
Here are some links with more information.

https://www.gardeningknowhow.com/ornamental/flowers/calendula/growing-calendula.htm
https://edis.ifas.ufl.edu/fp087

Calendula seed is easy to collect. Store in a glass jar once it is thoroughly dry. Here is a trove of seed pictures. https://www.google.com/search?q=calendula+seed&client=safari&rls=en&source=lnms&tbm=isch&sa=X&ved=0ahUKEwisxZ3OzcXcAhWJ5YMKHSiHCvYQ_AUICigB&biw=1391&bih=766

We don't usually suggest places to purchase, or specific products. I would check online stores such as Amazon, Etsy, or Ebay.

What I can do is offer out entire collection of articles pertaining to growing the plant: https://www.gardeningknowhow.com/category/ornamental/flowers/calendula