This achievement will require a bit of planning in order to avoid unnecessary grinding after earning Perfect Score all 25 levels of a difficulty mode. Nell's shop is where you head after the Upgrade Shop, before starting each level. Nell (Sally's friend) will give you the lowdown on the current demand for spa products. You will then be able to choose which products you want to put out to sell for the day. Focus on putting out the most expensive products that are popular that day. Use the dual-catergory products (ones with multiple colored symbols on them) only if both categories are popular. It is a good idea to upgrade the product shelf whenever possible to increase your sales as your customer base expands when progressing through the Story.
All of your product sales will add up across all levels and attempts of each level (within each difficulty mode). Once you approach $3,500 in sales, you will reach the highest rank in Nell's Shop, called "Best Friends Forever." If you do not reach best rank by the time you earn Perfect Score on all 25 levels, keep replaying the last level over and over until you sell enough products to earn the rank.