You can add it on the Document or Line Level.

Elements

Document level

Use the AllowanceChargeExtension (Extn) element to specify a charge on the Document level:

Element	Cardinality	Description
Allowance/Charge Group (`Invoice/AllowanceCharge`)	0..n	Groups information about Allowances and Charges.
Charge Indicator (`Invoice/AllowanceCharge/ChargeIndicator`)	1..1	True or false flag that indicates if this element is modeling an allowance or a charge. If set to false, it is an allowance. If set to true, it is a charge.
Allowance/Charge - Reason Code (`Invoice/AllowanceCharge/AllowanceChargeReasonCode`)	0..1	Code that specifies the reason for the allowance or charge. For countries without specific required values; you must use the UNCL 5819 codes listed here for allowances and the UNCL 7161 codes listed here for charges.
Allowance/Charge - Reason (`Invoice/AllowanceCharge/AllowanceChargeReason`)	0..1	Text that describes the reason the Allowance or Charge is permitted.
Allowance/Charge - Percent (`Invoice/AllowanceCharge/MultiplierFactorNumeric`)	0..1	The percentage that may be used, in conjunction with the document-level allowance base amount, to calculate the document-level allowance or charge amount.
Allowance/Charge - Amount (`Invoice/AllowanceCharge/Amount`)	1..1	The amount of the charge or allowance.
Allowance/Charge - Base Amount (`Invoice/AllowanceCharge/BaseAmount`)	0..1	The base amount to which a charge is added or an allowance is subtracted.
Allowance/Charge -Tax Category (`Invoice/AllowanceCharge/TaxCategory`)	0..n	A group of elements providing information about the tax category of the allowance or charge.
Allowance/Charge - Tax Category Code (`Invoice/AllowanceCharge/TaxCategory/ID`)	1..1	The codes that represents the Tax Category for the allowance or charge.
Allowance/Charge - Tax Rate (`Invoice/AllowanceCharge/TaxCategory`)	0..1	The percent tax rate that applies to the allowance or charge.
Allowance/Charge - Tax Exemption Reason Code (`Invoice/AllowanceCharge/TaxCategory`)	0..1	A code that represents the reason for an exemption for the allowance or charge.
Allowance/Charge - Tax Exemption Reason Text (`Invoice/AllowanceCharge/TaxCategory`)	0..1	A text that describes for an exemption for the allowance or charge.
Allowance/Charge - Tax Scheme Group (`Invoice/AllowanceCharge/TaxCategory`)	1..1	Groups information about the Tax Scheme that applies to the charge or allowance.
Allowance/Charge - Tax Scheme Code (`Invoice/AllowanceCharge/TaxCategory`)	1..1	The code that represents the Tax Scheme.

Line level

Use the InvoiceLine/AllowanceCharge element to specify a charge at the line level:

Element	Cardinality	Description
Invoice Line Allowances and Charges (`Invoice/InvoiceLine/AllowanceCharge`)	0..n	Groups information about Allowances and Charges.
Invoice Line Charge Indicator (`Invoice/InvoiceLine/AllowanceCharge/ChargeIndicator`)	1..1	True or false flag that indicates if this element is modeling an allowance or a charge. If set to false, it is an allowance. If set to true, it is a charge.
Invoice Line Allowance Charge Reason Code (`Invoice/InvoiceLine/AllowanceCharge/AllowanceChargeReasonCode`)	0..1	Code that specifies the reason for the allowance or charge. For countries without specific required values; you must use the UNCL 5819 codes listed here for allowances and the UNCL 7161 codes listed here.
Invoice Line Allowance Charge Reason (`InvoiceLine/AllowanceCharge/AllowanceChargeReason`)	0..1	Text that describes the reason the Allowance or Charge is permitted.
Invoice Line Percentage (`Invoice/InvoiceLine/AllowanceCharge/MultiplierFactorNumeric`)	0..1	The percentage that may be used, in conjunction with the document-level allowance base amount, to calculate the document-level allowance or charge amount.
Invoice Line Amount (`Invoice/InvoiceLine/AllowanceCharge/Amount`)	1..1	The amount of the charge or allowance.
Invoice Line Base Amount (`Invoice/InvoiceLine/AllowanceCharge/BaseAmount`)	0..1	The base amount to which a charge is added or an allowance is subtracted.

Restrictions

You must use the UNCL 7161 codes listed here.
For Charges, you must set the ChargeIndicator to true. If you do not, the AllowanceCharge is treated as an allowance. See Allowances.

Country-Specific configurations

The following country variations exist:

Example

Document level charge

<Invoice>
    <!-- code omitted for clarity -->
    <cac:AllowanceCharge>
        <cbc:ChargeIndicator>true</cbc:ChargeIndicator> 
        <cbc:AllowanceChargeReasonCode>FC</cbc:AllowanceChargeReasonCode>
        <cbc:AllowanceChargeReason>Freight service</cbc:AllowanceChargeReason>
        <cbc:MultiplierFactorNumeric>20</cbc:MultiplierFactorNumeric> 
        <cbc:Amount currencyID="EUR">200</cbc:Amount> 
        <cbc:BaseAmount currencyID="EUR">1000</cbc:BaseAmount> 
        <cac:TaxCategory>
            <cbc:ID>S</cbc:ID>
            <cbc:Percent>25</cbc:Percent>
            <cac:TaxScheme>
                <cbc:ID>VAT</cbc:ID>
            </cac:TaxScheme>
        </cac:TaxCategory>
    </cac:AllowanceCharge>
    <!-- code omitted for clarity -->
</Invoice>