I had opened a Case about this, There was not a satisfactory resolution(it would have involved a lot of coding).

Basically, It is unable to overflow at the Line level, only Package level.

Code:
<SHIPMENT>
	<ORDER_NUMBER>TEST090226001</ORDER_NUMBER>
	<PACKAGE>
		<BOX_NUMBER>1</BOX_NUMBER>
		<LINE>
			<LINE_INDEX>1</LINE_INDEX>
			<PRODUCT_ID>PRODUCTA1</PRODUCT_ID>
		</LINE>
		<LINE>
			<LINE_INDEX>2</LINE_INDEX>
			<PRODUCT_ID>PRODUCTA2</PRODUCT_ID>
		</LINE>
		<LINE>
			<LINE_INDEX>3</LINE_INDEX>
			<PRODUCT_ID>PRODUCTA3</PRODUCT_ID>
		</LINE>
		<LINE>
			<LINE_INDEX>4</LINE_INDEX>
			<PRODUCT_ID>PRODUCTA4</PRODUCT_ID>
		</LINE>
		<LINE>
			<LINE_INDEX>5</LINE_INDEX>
			<PRODUCT_ID>PRODUCTA5</PRODUCT_ID>
		</LINE>
		<LINE>
			<LINE_INDEX>6</LINE_INDEX>
			<PRODUCT_ID>PRODUCTA6</PRODUCT_ID>
		</LINE>
		<LINE>
			<LINE_INDEX>7</LINE_INDEX>
			<PRODUCT_ID>PRODUCTA7</PRODUCT_ID>
		</LINE>
	</PACKAGE>
	<PACKAGE>
		<BOX_NUMBER>2</BOX_NUMBER>
		<LINE>
			<LINE_INDEX>1</LINE_INDEX>
			<PRODUCT_ID>PRODUCTB1</PRODUCT_ID>
		</LINE>
		<LINE>
			<LINE_INDEX>2</LINE_INDEX>
			<PRODUCT_ID>PRODUCTB2</PRODUCT_ID>
		</LINE>
	</PACKAGE>
</SHIPMENT>