You placed your order, but you realized too late that you forgot to add an item. No problem, just create a new order and select "Add to existing order" as shipping option. No extra charges apply for shipping in that case. This option is available as long as the order is processing. After it is packed we cannot add products to it anymore. Make sure the weight/size of the combined orders does no exceed the limit of what is paid for, for example if you placed your order with shipping through untracked mail, you cannot order a spool of filament and it may cause delay in shipping your order. If in doubt please contact us.