I used an AI browser agent for this desktop Chrome UI check, after reading your description, so please treat this as a UI check rather than a blind human playtest. I completed Two roads, A round trip and Out of order (3/7), then stopped for the time limit.
The first puzzle’s failed check clearly explained that the parcel missed the stamp and reached the wrong-symbol mailbox. Routing it through the stamp then gave 1/1, with an explanation of the symbol change. In Out of order, the initial failure selected parcel 1 and left the later parcels marked not dispatched; after planning the alternating paths, the corrected setup delivered 4/4. Keeping the queue labeled STARTING SYMBOLS alongside a separate starts → arrives route summary was useful.
One specific copy improvement: “Blue A/B change exits after each parcel” could imply that both arrows flip for every parcel, including parcels that never visit B. “A blue arrow flips when a parcel passes through it; its state carries into the next parcel” would make the local rule clearer and match the second puzzle’s explanation.
I have not tested puzzles 4–7, mailbox capacity or the full difficulty curve.