The mid-band, revenue between 10 and 50 million
The clearest fit here. Large enough that someone's actual job is digital fundraising, small enough that the same person can decide without a procurement process, and almost never large enough to build in house. Also the band most likely to be running a checkout chosen by someone who has since left.
Who signs: the director of development, the digital fundraising manager, the marketing or communications director, and the chief operating officer on the contract.