In Odoo, you can group multiple SKUs or barcodes for a product under a single product line on an invoice by creating a product variant for each SKU or barcode and configuring the variant's attributes.

To do this, you would first need to create a product in Odoo for the "ALPHABET MUGS" product, and enter the necessary information, such as the name, description, and price. Then, you would need to enable the "Product Variants" feature for the product by going to the "Inventory" tab and checking the "Product Variants" box.

Next, you would need to create a product variant for each of the 26 variations of the "ALPHABET MUGS" product, using the appropriate SKU or barcode for each variant. To do this, go to the "Variants" tab for the product, and click on the "Add a Variation" button. In the "Variation Attributes" section, you can select the attributes that you want to use to differentiate the variants, such as the letter of the alphabet. Then, enter the SKU or barcode for each variant in the "Barcode" field.

Once you have created all of the product variants, they will be grouped under the main "ALPHABET MUGS" product in the product list, and you can select the appropriate variant when creating an invoice for a customer. The invoice will only show a single product line for the "ALPHABET MUGS" product, and the variant's attributes, such as the letter of the alphabet, will be shown in the invoice line details.