Q277: I want to change the way the $(reservation_details) tag works

You can't change the output of the $(reservation_details) tag, it's a tag which displays complex info about a reservation. You can however output the same information by using other tags from Q204. Below is an estimate which will print very similar information to the $(reservation_details) tag. You can use it as a basis for your custom output. $(if) $(total_resource_quantity) greater than 1 $(then) $(endif) $(if) $(price_quoted) greater than 0 $(then) $(if) $(online_payment_surcharge) greater than 0 $(then) $(endif) $(if) $(amount_paid) greater than 0 $(then) $(endif) $(endif) $(if) $(coupon_used) $(then) $(endif) $(foreach:$(custom_property_count)) $(endfor) $(additional_products_include_hidden) $(foreach:$(additional_product_count)) $(endfor) $(if) $(user_notes) $(then) $(endif)

$(S_RENTALID)	$(reservation_id)
$(S_RESNAME)	$(resource)
$(S_QUANTITY)	$(quantity)
$(S_DURATION)	$(rental_duration)
$(S_CREDATE)	$(creation_date)
$(S_STATUS)	$(status)
$(S_PRICE)	$(price_quoted) $(if) $(discount) greater than 0 $(then) ($(S_DISCOUNT): $(discount)) $(endif)
$(S_ONLINE_SURCHARGE)	$(online_payment_surcharge)
$(S_TOTPAID)	$(amount_paid)
$(S_COUPON_USED)	$(S_COUPON)
$(custom_property_name:$(it))	$(custom_property_value:$(it))
$(additional_product_name:$(it))	$(additional_product_price:$(it)) * $(additional_product_quantity:$(it))
$(S_NOTES)	$(user_notes)