It must be possible, to change the carrier at least until the products are shipped.
Could the checkout steps order change be a solution? For instance, customers pays for an order and after that he’ll be asked to choose and pay for delivery?
In that case it is possible to set the ‘default’ carrier, which will be applied to the order payed by customer in case when he/she refuse/forget to select appropriate carrier during reasonable period of time.
Anyway, the possibility to choose the carrier after order being payed (until it shipped at least) is good option. In Russia virtually all ecommerce companies allow delivery method changing after order been payed (NOT thru website, but via phonecall/messengers/emails instead)
Five minutes later the same customer places another order. We as a merchant merge the two orders into one
Should it merge be agreed with customer? What if he made 2 or 3 small orders intentionally? As far as i know, in PS/TB there’s feature for customer to “split” order and deliver its parts to different addresses, so the reverse operation could be easy as well - don’t merge some orders, but merge its delivery instead.
Products Returns: … if he only returns some items, it gets tricky. Too tricky. I can’t regenerate the invoice in that case.
As far as i know, it is not a big problem if customer returns some part of order - a return order should be generated then. IMO, worst problem will be when customer intends to return a part of product pack. What should i do as a seller in that case? Require returning of all pack, or just its defective/wrong parts? How to handle it?
IMO first we have to consult bookkeepers/accountants because this strongly depends on local bookkeeping rules/legislation and in could be virtually impossible to find out the rules applicable globally.