Going forwards, the best way to proceed when listing any items that you are not listing as Collection In Person Only would be to set the listing up with a number of different postage options before you even submit the listing, ideally with tracking in case an eBay Item Not Received case is opened against you and you need the tracking details to prove successful delivery to the buyer's address to ensure that eBay close the case in your favour. Possible options could include Royal Mail Second Class Signed For, Royal Mail First Class Signed For and Royal Mail Guaranteed Next Day Delivery By 1:00pm. If you wish to sell to international buyers as well then you could also include an overseas postage option, such as Royal Mail International Tracked And Signed.
Should you decide to follow this advice you would then have a listing with postage options that offer potential buyers a number of postage options. That way the buyer could just accept the standard Royal Mail Second Class Signed For option if he/she didn't need it too soon and/or didn't want to pay more in postage costs, although if the buyer needed the item sooner then he/she would also have either the Royal Mail First Class Signed For option or the Royal Mail Guaranteed Next Day Delivery By 1:00pm option to choose from. By making these options available when setting up the listing you don't have to try and find a workaround option to make them available after the buyer has made his/her mind up to buy the item.
As far as I am aware there are no postage options that would allow an international buyer to upgrade to a faster postage option, although the various other postage options that I mentioned should cover you for any buyers within the United Kingdom who may wish to upgrade to a faster postage option.
