For a 1-inch diameter roller tube on a Valu-Line RV roller shade that is 37 inches wide, the appropriate replacement spring is the Large Spring with a 24-inch total length, designed for shades 30 inches and wider. However, if your current spring is smaller and fits a 0.75-inch diameter roller tube, you should verify the roller tube diameter to ensure the correct spring size. Measure the diameter of the roller tube (not the spring) where the spring fits to confirm the exact size needed. If the widest part of your spring measures approximately 0.75 inches, then the smaller spring is the correct option for your shade. Regarding the return of the incorrect part, Elkhart RV Parts will waive any shipping or restocking fees since the purchase was made based on their advice. You can use the provided return label to send back the wrong spring, and once the return is received, the correct spring will be shipped to you promptly. If you need further assistance with measurements or installation, their support team is available to help ensure you receive the right part.