When the lyophilized stock material of a product is running low, a new batch (10 - 50 units depending on the frequency of sale) is lyophilized. Each new batch undergoes quality control testing. Once the previous lot is sold out, the new lot then becomes available for sale.
Due to this schedule, all products are always on stock and ready for sale. Hence, it is not possible to choose among different lots. Buying the same lot again is only possible if that lot is not yet out of stock.