Each shopper is unique. For some people, it might be advisable to keep the mystery shopping business separate from the rest of the income and tax situation. For others, it may work well to combine everything and use Schedule C, etc.
@OP: You are the only person who knows and who should know all the relevant factors in your decision. The posts here have shown that people find it easy to use Schedule C for sole proprietorships. We cannot go beyond that because we do not know all the relevant factors for you. If you have any questions whatsoever about the appropriateness of anything related to your tax situation, please consider consulting an actual tax pro who can provide specific information. You and your business are worth it.
Nature does not hurry, yet everything is accomplished. - Lao-Tzu