It may very well be the case that the organization has a policy for standard minimum reimbursement amount, and your expenditure was lower than that threshold. So you get the minimum amount ($100) as per the company norms.
Also, are you certain that they did not mention any reimbursement (with or without the charge slips / invoice) for your time (for travelling on-site) and refreshments? Some organizations do provide a token amount as a gratitude for the time you invested, over and above the actual claim amount.
If you're sure they did not mention any additional payment, over and above the amount your claimed, just to be clear from your side, you can drop an email mentioning the over-payment so that they are aware of the incident, if indeed this was a mistake. There's no harm in coming clean.
Most likely, you'll get to keep the card and the amount, as it does not soundappear to be a mistake, rather as per the process.