Interviews and hall tests can also be used for the documentation of user/consumer acceptance. This test method does not give you as much information as the other proposals. But do you already know that your limit for further reductions is user/consumer acceptance and "only" work for getting documentation interviews are a cost-efficient method. The test method is a quantitative method giving percentages to specific questions and not qualitative as focus groups investigating the user/consumer situation.
Do you already know that you are limited by?
- Ergonomics in opening the package with focus on disabled and elderly consumers
- Tamper evidence (security that the package have not been opened)
- Child-resistance
- Emptying in portions
- Re-closing the package for storage
- User convenience
- Etc.
And "only" need documentation; you can make a little hall test using interviews.