You could add a button with a rule that sets the value of TotalAmounts to the
value of a formula like

sum(Amount)

where "Amount" is the field in your repeating section. Another possibility
is to set the value of TotalAmounts whenever an Amount is filled in. You'll
have to use a helper field for the latter case. I did something similar using
the avg() function. You can check out that solution at:
http://enterprise-solutions.swits.net/infopath/avg-function-infopath.htm.
Instead of avg() you would have to use sum().
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S.Y.M. Wong-A-Ton