CsvImport.cs

using System.Globalization;
using System.Text;
using CsvHelper;
using CsvHelper.Configuration;

var sourcePath = @"C:\Users\nramsbottom\Desktop\Food Shopping.csv";

var reader = new StreamReader(sourcePath, Encoding.UTF8);
var parser = new CsvReader(reader, new CsvConfiguration(CultureInfo.InvariantCulture)
{
    HasHeaderRecord = true
});

parser.Context.RegisterClassMap<ReceiptLineMap>();

var receiptLines = new List<ReceiptLine>();

while (await parser.ReadAsync())
{
    receiptLines.Add(parser.GetRecord<ReceiptLine>());
}

var groupedByStore = receiptLines.GroupBy(x => x.StoreName);
var groupedByReceiptNumber = groupedByStore.GroupBy(x => x.GroupBy(y => y.ReceiptNumber)).ToArray();

foreach(var y in groupedByReceiptNumber)
{
    Console.WriteLine($"{y.Key}");
}

var x = 1;


class ReceiptLine
{
    public string ReceiptNumber { get; set; }
    public DateTime Date { get; set; }
    public string StoreName { get; set; }
    public string ProductName { get; set; }
    public decimal Price { get; set; }
    public string Currency { get; set; }
    public decimal? Quantity { get; set; }
    public decimal? UnitPrice { get; set; }
    public string? UnitSize { get; set; }
    public int? TaxPercent { get; set; }
}

class ReceiptLineMap : ClassMap<ReceiptLine>
{
    public ReceiptLineMap()
    {
        Map(x => x.ReceiptNumber)
            .Index(0);
        Map(x => x.Date)
            .Index(1);
        Map(x => x.StoreName)
            .Index(2);
        Map(x => x.ProductName)
            .Index(3);
        Map(x => x.Price)
            .Index(4);
        Map(x => x.Currency)
            .Index(5);
        Map(x => x.Quantity)
            .Index(6);
        Map(x => x.UnitPrice)
            .Index(7);
        Map(x => x.UnitSize)
            .Index(8);
        Map(x => x.TaxPercent)
            .Index(9);
    }
}