providing a sample of the XML you are sending would be helpful.
When adding them you do not need to provide ID's SAP will Assign them automatically... You just need the Product and quantity and pricing information
for example
<Item>
<ReceivablesPropertyMovementDirectionCode>2</ReceivablesPropertyMovementDirectionCode>
<Product>
<InternalID>MCF-0001</InternalID>
<TypeCode>1</TypeCode>
</Product>
<QuantityunitCode="EA">1.0</Quantity>
<QuantityTypeCode>EA</QuantityTypeCode>
<PriceAndTax>
<PriceComponent>
<TypeCode>7PR1</TypeCode>
<Rate>
<DecimalValue>500</DecimalValue>
<CurrencyCode>USD</CurrencyCode>
<BaseDecimalValue>1</BaseDecimalValue>
<BaseMeasureUnitCode>EA</BaseMeasureUnitCode>
</Rate>
</PriceComponent>
</PriceAndTax>
</Item>