Here are the statements we've received so far:
Air Canada: "Hopper is an on-line agency that sells Air Canada fares. As noted on the aircanada.com website, customers who have booked with a travel agent or any online travel agency must contact them directly for options as each partner has a unique booking system that we are unable to access in order to adjust bookings. The online agencies are able to offer the same options to customers, including: refunds for refundable tickets as per those ticket conditions, conversion of non-refundable tickets to travel credits or non-expiring vouchers or to Aeroplan points with bonus."
American Airlines: "If a ticket sold by Hopper is refund-requested via American Airlines channels (i.e. aa.com or Reservations) and the ticket is eligible, we refund back to the original form of payment, even if the credit card belongs to the third party. In the case where a travel agency sold the ticket and we are notified that the agency is no longer in operation or we can confirm that the agency is not refunding the customer, as long as we have the correct contact information, we refund the customer and attempt to recover the funds from the agency."
United: "Tickets purchased through third-party agencies must be submitted for refund directly to the agency, which sold the ticket to its customers and thus has the responsibility for issuing any refund that may be owed."
Delta: A representative told us Hopper has to comply with Delta's refund policies. If Delta cancels the flight, the customer is entitled to a refund on request. If the customer cancels, they can get an eCredit. Delta says Hopper, like any agency, is responsible for processing the refund to the customer.
WestJet: A representative told us "if a guest booked through a Travel Agent (online or directly), third-party like Hopper, Corporate Travel arranger, or another airline, they would have to contact them directly." The company noted its refund policies are here and that, on Oct. 21, 2020, it said it would provide refunds to all travelers if WestJet initiated the cancellation.
Virgin Atlantic: The company only responded that it wasn't affiliated with Hopper. We're awaiting a further response after an attempt to clarify the problem.
[We'll update if others respond. Some did not.]