You have to initialize grandTotal
like so:
<cfset grandTotal = 0>
<cfloop query="ActivityReceipts">
<tr>
<td style="text-align:left;">#ActivityReceipts.activity#</td>
<td style="text-align:left;">#ActivityReceipts.qty_approved#</td>
<td style="text-align:left;">#ActivityReceipts.location#</td>
<td style="text-align: right; padding-right: 80px;">
#ActivityReceipts.payment_amount#
</td>
</tr>
<cfset grandTotal = grandTotal + ( ActivityReceipts.payment_amount
+ ActivityReceipts.handling_cost
+ ActivityReceipts.Shipping_cost
) />
</cfloop>