Is there a non-religious alternative to the expression "mixed blessing"?

Solution 1:

I don't think that most people would think of it as more than just a regular saying, but if you really do want to avoid the word blessing, there are a few other expressions you might use. Some would require rephrasing your passage.

• was of mixed utility / sent mixed messages
• had its pros and cons
• was good news and bad news
• was a double-edged sword (This might connote violence.)
• was bittersweet (This might not be used in all the same contexts.)

In terms of the thinking of a person, you could also say that they were of two minds.

Solution 2:

Depending on your audience, you could say that the invitation was something of a silver lining for Charlie.

This expression invokes the proverb “Every cloud has a silver lining”, but it suggests that the bad (the cloud) is bigger than the good (the silver lining). It also suggests that the whole story is not being told, but that the intelligent reader can infer it from their broader knowledge.

A corresponding proverb is “Every rose has its thorn”, although possibly not so well known.

The best choice really depends on the tone you want to strike in your description of Charlie.