Just clip the $20 coupon on top of list price after login and apply it on checkout.
S50, S51, S55 all the same, just different color, S50(white) ,S51(white + rose gold), S55(black), you can get S50 or S55 with $10-20 surplus.
My own Roborock just updated newest firmware 3.3.9_001632 2 days ago, now it can save permanent map and place no-go zone + virtual wall on it. (see screenshots below)
Rose Gold: https://www.joybuy.com/product/600209376.html $350 after $20 coupon
White: https://www.joybuy.com/product/600136672.html $360 after $20 coupon
Black: https://www.joybuy.com/product/600248747.html $370 after $10 coupon
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