I always comply by putting your invoice in a special plastic document holder on the outside of the parcel, and I attach various informative stickers plus a Customs Declaration too.
I fill this out stating that the Nutritional Supplements are a commercial item, what they cost and their weight. This always seems to work just fine.
The responsibility for receiving the parcel is yours. This means that the parcel will have to be signed for. If it’s returned to me for any reason, you will be responsible for the cost of my having to send it out to you again.