Glass or plastic
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This reads like a decision about how the jar looks and what it costs to ship. For spice it is mostly a decision about light, and light is the thing that fades the products you most want to look good.
The decision is light, not strength
Carotenoid pigments, the ones that make paprika, chili, turmeric and annatto what they are, oxidize and fade under light. A clear container accelerates that; an amber or opaque one slows it substantially.
Spice is not hot filled, it is not acidic, and it is not heavy. Almost every reason that drives a material decision in other food categories is absent here, which leaves light and moisture barrier doing the deciding.
What each material actually does
Clear glass displays the product, which is the strongest sales argument in the category, and offers no light protection at all. It is an excellent moisture barrier.
Amber glass keeps the moisture barrier and blocks a large part of the damaging light. It costs more, it is less visually immediate, and it is the right answer for anything pigmented that will sit on a shelf for months.
PET is light, cheap and shatterproof, which matters for markets and for shipping. It is a poorer moisture barrier than glass, which over a cupboard's worth of months is a real difference.
Tin and opaque block light completely and hide the product entirely, which suits a brand selling on identity rather than on contents.
Cost and weight
Glass is several times the shipping weight of PET for the same volume, and shipping weight is most of the delivered cost of an online order at these unit prices. A four ounce glass jar can cost more to post than to fill.
Deciding
Clear glass for fast-turning lines sold face to face, where the display sells and the product does not sit. Amber or opaque for pigmented spices and for anything sold online into a cupboard. PET where breakage or postage dominates, accepting the shorter effective life.
And pick for the most vulnerable member of the range if the range has to look consistent, rather than for the average.
