This Wikipedia page lists a number of separate allegations over the past 15 years that Interpal has ties to, and raised funds for, Hamas. The page claims that none of the allegations have been proven.
However, the Hamas-leaning newspaper Palestine Times shows images of (unnamed) members of the convoy meeting with Hamas leader Ismail Haniyeh in Gaza.
I don't know if any Interpal members are in these photos but it is worth looking further into.
There are two other things that indicate that Interpal is not quite apolitical as it claims.
One is that its logo includes a map of all of Israel:
And another is that on its webpage, for no apparent reason, they have a section about the "Al Aqsa Mosque" that claims that it is not a building but the entire Temple Mount, seemingly including the Western Wall.