DataGridComboBox not returning value from object

I'm fairly new to WPF and still trying to understand the bindings. I've been successful a few times, however I am struggling here.

I am Binding a Datagrid to a List<> type object. 2 of my datagrid columns bind successfully, however I'm struggling to bind my DataGridComboBoxColumn.

My DataGrid is as follows:

        <DataGrid Name="CustomerList" AutoGenerateColumns="False" AlternatingRowBackground="AliceBlue" ItemsSource="{Binding Source=customers}">
            <DataGrid.Columns>
                <DataGridTextColumn Header="BP Code" Binding="{Binding Path=CardCode}"/>
                <DataGridTextColumn Header="BP Name" Binding="{Binding Path= CardName}"/>
                <DataGridComboBoxColumn x:Name="EDIcombo" Header="EDI Format" ItemsSource="{StaticResource sEdiFormat}" DisplayMemberPath="Key" SelectedValuePath="EdiFormat"/>
            </DataGrid.Columns>
        </DataGrid>

This pulls static values for the selectable options on the drop down- this part works fine and I get my list of values to choose from, however I don't have the default value that should be pulled from the 'Selected Value Path'

My List object is as follows:

public class EDICustomers
{
    public string CardCode { get; set; }
    public string CardName { get; set; }
    public string EdiFormat { get; set; }

    public EDICustomers(string cardcode, string cardname, string ediFormat)
    {
        this.CardCode = cardcode;
        this.CardName = cardname;
        this.EdiFormat = ediFormat;
    }
}

My method that runs sets the ItemSource of the Datagrid to be the List<> object in question. I'm confident that the issue is with the binding of the SelectedValue.