Cache methods on Manager.

 Project Kavo 

ExtendedProductCategoryManager

/// Ticket 88704: [Kavo] 3.9. PDP – RELATED ITEMS
        /// <summary>
        /// Get product categories by product child level.
        /// </summary>
        /// <param name="level">product's child level</param>
        /// <returns></returns>
        public virtual IProductCategoryCollection GetProductCategoriesByLevel(int level)
        {
            bool fromCache;
            var categories = CacheManager.FromCache(ErpCollectionCacheKey<IProductCategory>() + "CategoriesByLevel" + level,
                    () => ((ExtendedOfflineProductCategoryProvider)Provider).GetProductCategoriesByLevel(level),
                    out fromCache);
 
            return categories;
        }

use a unique key for cacheKey 

/// Ticket 88704: [Kavo] 3.9. PDP – RELATED ITEMS
/// <summary>
/// Get related product item.
/// </summary>
/// <param name="categoryId"></param>
/// <returns></returns>
public virtual IRelatedProductSet GetRelatedProductSetByCategoryId(string categoryId)
{  
   var cacheKey = CacheKey.ForContentCollection<IRelatedProductSet>(categoryId);
   var list = CommerceFrameworkBase.Content.GetContentCacheContainer()
       .FromCache(cacheKey, () => ((ExtendedOfflineProductProvider)Provider).GetRelatedProductSetByCategoryId(categoryId));
        return list;
}

cache with List exampe. 

ExtendedProductManager

/// Ticket 88704: [Kavo] 3.9. PDP – RELATED ITEMS
/// <summary>
/// Get related product items by category ids.
/// </summary>
/// <param name="categoryId">category id</param>
/// <returns>related product item</returns>
public virtual IList<IRelatedProductSet> GetRelatedProductSetsByCategoryIds(List<string> categoryIdList)
{    
    string categoryIdListString = string.Join(" ", categoryIdList.ToArray());
    var cacheKey = CacheKey.ForContentCollection<IRelatedProductSet>(categoryIdListString);
    var list = CommerceFrameworkBase.Content.GetContentCacheContainer()
        .FromCache(cacheKey, () => ((ExtendedOfflineProductProvider)Provider).GetRelatedProductSetsByCategoryIds(categoryIdList));
    return list;                       
}

another simple way to cache

ExtendedCommonManager

public IPIMModelsDataCollection GetPIMModels(string makeId)
{
var key = CacheKey.ForContentCollection<PIMModelsData>() + "PIMModels" + makeId;
return CacheManager.FromCache(key, () => ((ExtendedOfflineCommonProvider)Providers.Offline.Common).GetPIMModels(makeId));
}

public IPIMMakesDataCollection GetPIMMakes()
        {
            var key = CacheKey.ForContentCollection<PIMMakesData>() + "PIMMakes";
            return CacheManager.FromCache(key, () => ((ExtendedOfflineCommonProvider)Providers.Offline.Common).GetPIMModels());
        }