The expense nobody mentions can keep a supposedly simple invitation difficult to accept.

Invitations carry hidden bills. A casual suggestion to join for dinner includes the travel, the round of drinks, the babysitter, the clothes that would be expected, and none of that appears in the sentence asking you to come. For some people the sum is nothing and for others it is a week’s slack, so the same invitation is easy and impossible depending on who receives it. Declining then looks like reluctance rather than arithmetic.

When you invite, say the cost out loud and make the cheap version genuinely available. And if you are the one declining, you owe nobody the details.