EBay do not restrict your selling and drop you to Below Standard for not offering free postage. Plenty of people don't offer free postage and are top rated sellers and power sellers. If you check your seller dashboard you will probably find you have received too many defects.

Have you given any refunds recently? Depending on the reason you give, you will get a defect for refunding. If people give you neutral or negative feedback, you get a defect. If you are given low star ratings by buyers, you get a defect.