tl;dr: For low value orders like these, only a general description is usually needed to get most products through customs.
I have a little insight on shipping as I wrote the shipping software for the company I work for. So I do not deal with the complexities of shipping every day but I know some of the inner workings of shipping, especially with UPS and FedEx.
Note: I’m not experienced with the shipping of anything food related.
To ship something between countries you need a little more information than shipping domestically.
This is usually more precise descriptive information of what you are shipping.
For shipments with a single commodity at or above a $2500 value, an EEI/SED form is required. These are often filed for orders that total $2500 or more as well, as they can smooth passage through customs.
These forms detail what is being shipping and include Harmonize Codes to describe what is being shipped.
A couple codes that could be valid for Soylent: Food Supplements Harmonized Code Supplement & Protein Powder Harmonized Code
However, for low priced shipments like what an individual might have for Soylent, at most all that is required beyond domestic shipments is a simple description of what is being shipped in general, in this case something like ‘Food Supplement Powder’ would work.
Each country does have restrictions on certain imports or imports worth over a certain amount.
Most of the cases I’m familiar with involve imports above a certain value rather than of specific types.
Customs tends to be most concerned that any invoice paperwork you include is matching what is in the box than anything else.