The answer below is no longer valid as FlixBus has changed their carriage conditions and no longer allows e-scooters.
According to the Terms and Conditions of Carriage,
22.214.171.124 E-scooters are considered special baggage, which means they are subject to the size and weight restrictions of a special baggage (c.f. clause 15.3.2) and are carried in the baggage compartment for an additional fee (c.f. clause 15.3.5). For transportation, the e-scooter must be folded up and packed in a suitable bag.
Other relevant provisions:
15.3.1 So-called special baggage requires prior registration by passengers and confirmation that its carriage is possible. No general claim to the carriage of special baggage shall exist.
15.3.2 Items that exceed the standard baggage dimensions are considered special baggage. However, the overall dimensions of special baggage items may not exceed 240 cm (height in cm + width in cm + depth in cm). A single item of special baggage may not exceed a maximum weight of 30 kg.
15.3.3 The carriage of special baggage shall be limited to one item per passenger.
15.3.5 Special baggage must be registered in advance, either (if it is possible for the trip) via the booking system or by telephone, at the
earliest 48 hours before the start of the trip, via the following
Calls from Germany and Switzerland (German landline): +49 30 300 137 300
Calls from Austria (Austrian landline): +43 820 910 340
Calls from Italy (Italian landline): +39 (02) 947 59 208
Calls from France (French landline): +33 (0)1 76 36 04 12
Calls from Great Britain (British landline): +44 1 78 82 98 784
Calls from Croatia (Croatian landline): +385 1 800 03 80
Calls from Denmark (Danish landline): +45 32 72 93 86
Calls from Sweden (Swedish landline): +46 850513750
Calls from Poland (Polish landline): +48 22 307 93 34*
Calls from Russia (Russian landline): +7 (495) 283-97-30
If special baggage is transported, an additional baggage fee per item
will be charged. The fee for a special baggage is regardless of the
length and ticket price of the route booked. The transportation of
special baggage, in contrast to normal baggage, is always subject to a