We ship everything on EVRi next day service - now granted not everything arrives next day however we do 92% according to amazon shipping metrics

but our delivery times are now showing as 48 hour delivery rather than 24hour on products

B0048AKB8Y

for example

how can we change this? or is it simply fact we only have 92% on shipping metrics, and this is clearly EVRi's fault for poor service - but Royal Mail 24 is nowhere near as reliable and costs £1 more per parcel.