Image The Publican drew its to-go menu from its greatest hits, including barbecued carrots. Credit... Joshua Lott for The New York Times

Prices have to adjust, too. The Publican is boxing up enough food for four people in meals drawn from what its website terms “greatest hits,” like piri-piri chicken, summer sausage, fries, salad, barbecued carrots and cookies, all for $16.95 a person.

“Depression-era pricing,” Mr. Kahan said. “We just want to keep some cash flow going. Survival.”

Publican’s greatest hits can be purchased through Tock, which was a reservations site until about a week ago. Then Nick Kokonas, its founder and chief executive, got a call from Brian Canlis, an owner of the restaurant Canlis in Seattle.

“He said, ‘Hey, we need to do a pickup meal and I’m not sure I can properly configure enough of them as ‘tables,’ ” Mr. Kokonas recalled in an email. “He was trying to ‘hack’ the existing system, which is very powerful for normal restaurants but was not geared to carryout inventory.”

Tock wrote some new software so restaurants could sell meals online and, crucially, process credit card payments, which requires an ability to encrypt data that most restaurants do not possess. About 40 restaurants in the United States and several in other countries have started using the new service, Tock to Go.

By Sunday, a competing reservations service, Resy, which like Tock was watching the numbers of bookings on its site crash in one city after another, had also repurposed its site for delivery and pickup orders. About 30 restaurants have signed on. (Resy has waived its fees to restaurants for at least 30 days. Tock is collecting its monthly fee.)

Restaurants are the main clients for many other businesses, from florists to linen services to farms, that have lost income as people have been ordered to stay home. Few of them are as nimble, though, as the technology-driven reservation services, and even they are uncertain how many of the to-go operations that were put together over the past few days will still be running next week.