You may need to replace your extruder with a spare part (like-for-like replacement) if it develops a fault over time, or if you damage the unit accidentally. With good care, your extruder will last for thousands of print cycles but if you use it extensively this will reduce its lifespan, as with all mechanical components.
Levelling shims are another Markforged spare part we sell quite regularly. These exist to eliminate flatness issues when calibrating the print bed. They aid alignment and levelling to ensure the bed clicks into place right and are compatible with all Desktop series 3D printers and may be suitable for others (please ask).