You're better off saving $20 buying direct from Crosman with the AGN 25% off discount code. $28.50 order Friday to get free shipping Fridays.
40mm vs 50mm for a 3-9x shouldnt matter much.
https://www.crosman.com/optics/scopes/3-9x40-mm-centerpoint-tag-scope-4625
They (Crosman) sell rings for cheap and they are decent quality you may add to yiur order with the 25% code and free shipping Friday.
Be sure to get proper 1 inch rings and either 11mm or Weaver-Picatinny depends on your gun's scope rail adapter type.