1. The jump from 50 to 500 is literally a 10x jump. A seller who is teetering on 50 will need 250 at most for the next level. Charge $15 for 250 and then $30 for 500. It shows that your goal is to facilitate success with your customers and not "lock'em in" because they finally had some success.
2. $9 to $29 is a 3X jump. It might seem trivial to add $20 to the monthly fees, but every dollar counts for a company trying to stay afloat right now. I know that you have a prorated fee in place, but the admin around that just seems like upscale engineering.
3. Most importantly, sellers need a partner who has their back. Make a plan that works for them in that middle ground and I'm almost certain you'd be investing in a long-term customer. I love Shipstation, and I say this because I want this product to be that great.