Billboard tenant not paying late charge

[William Avery]

I have a billboard tenant that consistently pays late and per the terms of the agreement I have been adding a late charge. Questions:

1) Is it ok to accept the rent payment without the late payment? Should I send the check back and explain that I don't accept partial payments?

2) What recourse do I have if they continue to ignore the late charge? Pay an attorney a few hundred dollars to try to collect a few dollars?