What are the key factors to consider when choosing the best panniers for grocery shopping, and how do different types of panniers, such as single-compartment vs. multi-compartment, hard-shell vs. soft-shell, and various mounting systems, impact their performance in this specific use case. Additionally, what features should one prioritize when selecting panniers for grocery shopping, such as waterproofing, durability, and ease of loading and unloading. Are there any panniers that excel in this area, and what are the trade-offs between different designs and materials. How do the needs of grocery shopping differ from other types of cycling, such as commuting or touring, and how do these differences inform the choice of panniers. What is the optimal balance between capacity, weight, and maneuverability for grocery shopping panniers.