Value added tax payers authorised to have the additional period of one month provided for in 2 of Article 287 to submit the declaration referred to in 1 of the same article are required to pay, by way of a deposit, within the period stipulated in the said 1, a sum determined by them on the basis of their turnover, the taxes to which they are liable and the deductions to which they are entitled. This sum must be at least equal to 80% of the sum actually due. Any difference between the sum thus paid and that actually due is deducted from the advance payments for the following months, or an additional payment must be made at the same time as the taxpayers file their tax returns.