United offers subscription plans that will let you reserve an Economy Plus seat on any United or United Express flight that has available Economy Plus availability. Plans start at $499.00 and have options for customizing your subscription by region and companion travelers.
Delta was first and now other carriers are jumping on board offering basic economy fares. Booking tools are working to filter these fares out (depending on company policy) but in the meantime you should check the fare rules. These fares are non-refundable, cannot be changed and no seat assignments are allowed. In some cases there are also baggage restrictions. When booking online there are links to the fare rules under "Show Detail" or "Fare Rules". One big indicator that you may be booking these restrictive fares is when you proceed to select a seat(s), there are no options to reserve them. You should always be aware of the rules of the fare you are purchasing.