Just send them what you can afford, that is much better then nothing at all. When my first child was born I couldn’t afford the $2k bill but paid them a little each month for a couple of years. That was better than returning the merchandise (just a little humor thrown in)
Just keep in mind that your minimum payment needs to be above the interest threshold if you want to make headway on the loan.
Like Steve said, be sure to contact them and tell them what you can afford to do, in most cases they would rather receive something.
Repayment of debts is a sign of maturity and you do not want to have that hanging over your head. The feeling that “do I really have to repay this” coupled with the poor lending practices is what has put us in our current financial crisis. Always repay your loans, stick with it for the long run.