How can I aggregate (SUM) the amount?

To use an aggregation function, use the AggregateResult type. In addition, it is necessary to use valueof method for each type to convert the type, because the aggregation result data cannot be stored to a Decimal variable directly.

For specifics, refer to the sample below:

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AggregateResult ar = [SELECT sum(EntryCharge__c) s FROM AllocationTable__c WHERE id=:id];  
TotalCharges__c = decimal.Valueof(string.valueof(ar.get('s')));  
system.debug('data = '+String.valueof(j.TotalCharges__c));  
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For String type conversion, refer to this topic.