if the person to whom you are paying these expenses does not have gst, than it is your liability to pay tax on rcm basis....subject to day limit of 5000.....
whether goods are in transit or not does not matter if have purchased from a registered dealer....your liability will depend on the date of issue of invoice ( not the date of receiving of invoice or goods)....if invoice has been issued on 30 July than liability will arise in July itself.....(it is important to point out that time of supply is the date of invoice or date of payment whichever is earlier)....... now don't think it means that you can delay the payment by saying seller to issue invoice few days after.....as the date of invoice will be deemed in that case the last date on which invoice need to be issued under gst law....and in this case last date is when goods leave from the premise of seller...
Stationary expense also attract in RCM.... Sales promotion also applicable but, define full details...... then segregate which is taxable or non taxable......