In the UK, this would certainly be referred to as a a 'sachet'

I would disagree with those answerers who suggest that 'packet' is mainly a cardboard box, since the term is very often used for 'bagged' goods too.

I think if it is fairly flat, and probably fairly small, it suggests sachet to me; also, if it contains a powder or somewhat liquid product. A sachet may well be constructed of some kind of tougher material, like foil-backed plastic or paper (quite hard to tear...)

So you get a sachet of medicinal powders, a sachet of dried yeast, a sachet of gelatine, a sachet of gourmet cat food...

Where the pack is slightly larger, perhaps more 'puffed up' with air, and maybe containing individual items (rather than a powder, sludge or liquid), I would think of it forst and foremost as a packet:

a packet of crisps / boiled sweets / rice

A packet may also suggest an outer pack containing inner sub-packs(which might be sachets, or something else)

I am not familiar with any very frequent UK usage of 'packet' to mean a cardboard box or carton, though it does of course exist, in things like 'a ¼lb packet of loose tea', or of course, in the sense of a parcel: sending a small packet via the post.

Generally, 'packet' in this sense suggest to me simply something that is 'wrapped up'

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Note added at 3 days6 hrs (2006-03-01 18:01:24 GMT)
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Sorry, but no! In UK English at least, 'pouch' suggests something quite specific, not sealed, with a flap, the sort of thing loose pipe tobacco or documents might come in. Not at all appropriate for your kind of products.

It can also have a most unfortunate and inappropriate medical connotation....

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Note added at 3 days6 hrs (2006-03-01 18:03:03 GMT)
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If you really want to bridge the gap, just call it a packet; this will be understood OK without ambiguity in the UK, even though it probably wouldn't be the first term that would spring into most people's minds.