Excel help, adding financial equations incorrectly

[Kehl]

To solve some problems at work I tried creating an excel spreadsheet that an employee could enter the nightly figures in and it would calculate the fees charged by our credit card processing company, subtract that total, and then calculate a grand total. It seems to subtract the fees just fine and give the subtotal for each transaction type, but it is adding the grand total up using the thousandths decimal place and then rounding it to the hundredth. An example:

Discover:  $82.27
$82.27 - ($82.27 * 0.016) = $80.95
Visa:  $72.27
$72.27 - ($72.27 * 0.02) = $70.82
Total:  $151.78

Excel rounds the percentages right and subtracts everything perfectly, but when it totals the two numbers using =SUM I get $151.78 instead of $151.77. So I expanded the equations to the thousandths decimal place and got this:

Discover:  $82.27
$82.27 - ($82.27 * 0.016) = $80.954
Visa:  $72.27
$72.27 - ($72.27 * 0.02) = $70.825
Total:  $151.778

That seems to explain why it occasionally rounds up or down a penny, but does anyone have any suggestions on how I can prevent this from happening? It is throwing my monthly totals off a small amount and the idea that it will "all balance out" in the long run doesn’t work for me.