"More is welcome" or "More is welcomed"

Solution 1:

They're both grammatical:

• Welcome is an adjective which means agreeable or gladly received.

• Welcome is also a verb meaning to receive or accept with pleasure. Formed into a past participle, which functions as an adjective, it is welcomed.

More is welcome is definitely the one you want, since it implies to the viewer that they are being targeted by the statement. More is welcomed, on the other hand, sounds like a bare statement of fact that you currently accept more, without the same appeal to the individual viewer.

Solution 2:

• "More is welcomed" relates to the money being welcomed (greeted at the door), should it arrive.
• "More is welcome" relates to the concept of "more" being welcome (door is always open), should it arrive or not.

Also,

• the first implies that you intend to thank them when they give you money.
• the second is more in spirit with the sentence: "Thanks, door's open, just leave it on the table", thus not necessarily implying an immediate thank you.