Calibrachoa . . .
I know that Calibrachoa requires well-drained soil. I'd like to put it around my mailbox (in full sun), but I have clay soil. Would I still be able to plant it in that location if I took a shovel-full of clay out (for each plant) and filled the hole with good gardening soil? Also, what's the best way to keep the soil more acidic?
Adding sphagnum peat moss helps acidify but there are commercial soil acidifiers at home stores. Here is an article on your choices: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/raise-acid-level-soil.htm
What I would do concerning drainage is a percolation test. It is a several step process to check drainage under the planting hole. Add enough water to fill the hole and let it drain. Once drained, add enough water to fill the hole again. If the water isn't gone in 24 hours, further amending is needed. https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-soil-conditioner.htm